In this video, Steve Vai's guitar tech Doug explains the different degrees of scalloping that Vai uses for different guitars:

"Flo": 3:29

"And here's another old guitar, this is the original "Flo" which has made a reappearence. This is a newer neck, it's a stock Ibanez neck that we lightly scalloped, and yeah, Steve sort of prefers a light scallop these days, and EVO Gold fretwire which is the jumbo size, not too big, not too small... and yeah this is a very cool guitar, classic Vai."

"Flo III": 3:58

"And here's another workhorse, here's "Flo III" which has been Steve's main guitar for maybe the past decade, maybe ten, twelve years. Y'know stock, it has the Fernandes Sustainer, slightly scalloped EVO Gold frets..."

"PIA": 4:53

"Here's a newer main guitar, yeah this is one of the new PIAs, and the PIAs are really great, they feel excellent, Steve's really happy with them. These have stainless steel frets, which you can't wear them out, y'know they're very durable, and they have a very glassy feel for vibrato, and this guitar it's slightly scalloped, it has a Sustainiac, which is something we're still kinda exparamenting with, it's fantastic, but Steve's just getting used to them, because he's been using Fernandes [sustainers] for, 20+ years, yeah? This guitar we're using on "Giant Balls of Gold"."

"ENVY": 6:26, 7:15

This is sort of the sister to "BO", "ENVY"! which is another PIA. This guitar, it's setup a little differently, this one has the kinda offset L.E.D.s. Yeah, this one's setup for "Bad Horsie", there's a slide, Thomas' classic velcro-slide situation lock, which works great, it's fantastic, it's never failed. Again Sustainiac, y'know 'cause he uses it for those songs...

...This one's scalloped a little deeper, and has Jumbo EVO Gold frets.

"Bruno"/"For the Love of God" Guitar: 7:40

...Yeah, you'll recognise this from the "For the Love of God" music video. This is the 7-String guitar Steve is using on this tour for "Zeus in Chains" and it's setup with 9-52 gauge strings tuned to Drop-A and this is a really great sounding guitar pretty much stock...

...No scallop here yeah, the 7-Strings we don't scallop.

In the Ernie Ball: String Theory video featuring Joe Bonamassa, at 3:24, a scalloped fretboard can be seen on a Maple-neck Fender Stratocaster with a yellow/aged-white finish resting on a workbench. The presence of this guitar suggests Bonamassa's interest in or use of scalloped fretboards, although it's uncertain if this is a new model or a customized piece, possibly inspired by the Yngwie Malmsteen signature series.

Yngwie goes into detail about why he uses scalloped frets. There are also many other interviews he does explaining the same thing.

Blackmore has been widely noted as one of the early users of the scalloped fretboard, eventually inspiring guitarists like Yngwie Malmsteen to pick up the same idea, further popularizing it in the 1980's.

At the beginning of 1971 a new 71´s sunburst Strat showed up, sporting a four-bolt maple neck with skunk stripe, two-piece steel tremolo, stock pickups and electrics. Like on his old black Strat the tremolo was as well customized to his specification: a heavy custom-made tremolo arm made of steel was fitted and a bigger hole had to be drilled into the steel block to hold the fatter arm. The lacquer of the fretboard was peeled off right from the start. This is the reason why the fretboard looked darkened after only a short time, but the frets stayed untouched. It is possible that Ritchie already tried a rudimentary type of scalloping here. The legendary “Beat Club” session from September 1971 was recorded with this sunburst Strat. The new 71 Strat had a more airy and pearly attack, something that Ritchie really liked and fell in love with more and more. It was used in parallel to his old black one during 1971. In late 1972, this sunburst axe was rather damaged at its headstock. He played this sunburst Strat until early 1973 than smashed it. Blackmore himself said that he played scalloped fretboards since about 1971.

In this image, Downing can be seen performing live with his custom scallop necked Flying V. In a March 7, 2019 interview with full in bloom, Downing states that he's been using scalloped neck guitars since roughly 1984:

full in bloom: "Were you shocked when the ’67 Gibson Flying V sold for almost $200,000?"

K.K.: "Yeah (laughs). I was expecting, to be honest, about half that."

full in bloom: "I think on the (auction) site they estimated it to be around 10 to 20 thousand dollars."

K.K.: "Yeah, I think that’s probably just to get a massive audience. I don’t know how it works, myself. Obviously that was a 1967 Flying V and Gibson only made 111 of those guitars, and those guitars were put in some pretty famous hands, to be fair. I don’t know everyone that had one…I know Dave Davies from The Kinks had one. I know Jimi Hendrix had one. I know that Andy Powell from Wishbone Ash had one, still plays that guitar, he does. I know that Marc Bolan had one. I know that Keith (Richards) from the Rolling Stones had one. I do know where a lot more went, but there wasn’t that many to go around. Obviously I got my hands on one. Obviously the Schenkers had a couple. So they were bound to have been worth more than 20….I was offered $25,000 more than 20 years ago, cash there and then by a guy that went to a concert. That guitar was used on a lot of stuff, a lot of concerts. It was my flagship guitar, for sure. But I retired that guitar a long time ago because it was getting too valuable. So I’d never take it out on the road, and now I’m using scalloped frets and all of that sort of stuff. So, nice to pick up now and then but really there’s not a great value in that. I’ve still got some very, very nice and some very valuable guitars."

full in bloom: "You use the scalloped neck now, huh?"

K.K.: "I’ve been using the scalloped frets now since about 1984."

full in bloom: "Oh crap, I didn’t know that. (laughs)"

K.K.: "Yeah, yeah. Every guitar I’ve had since then has got scalloped frets."

full in bloom: "What would you say is the benefit of that? I have played on them before but it almost seems like it’s harder or something."

K.K.: "Yeah, but if you’re a little guy like me, not very strong hands and all that, then scalloped frets can really help. I use light gauge strings but it still takes a bit of pushing, here and there, really. Especially if you want to get those tone and half bends in. In particular, the main reason, especially when you still have got soft skin…I used to get calluses on my fingertips but I don’t really get calluses anymore because you’re using light gauge strings and scalloped frets. I find I can make those bends a lot easier, because, the thing is, you get fingertip friction and that’s why a lot of guys went to jumbo frets, you know, to get less finger friction. There’s not that much friction with the finger and the fretboard, so that does help. You can get your finger more sideways on the string when you’re bending, when you push it, as opposed to having to push down and then laterally. So, it’s just one of those things. Once I tried it, I just liked it and never went back. Way back we’d still play hot and sweaty clubs and so doing those bends just became a lot easier with scalloped frets, really….a bit more reassuring."

[...]

-full in bloom

In this photo, Roth can be seen playing his signature Sky Guitar, complete with a 30+ fret scalloped fretboard, this guitar appears to be scalloped from the 6th fret onward, with the first five frets appearing to be unscalloped for heavier chording & riffing.

Wikipedia also notes Roth's use of scalloped necks in his signature model guitars:

"Sky guitars" "Roth commissioned construction of custom guitars with additional frets from luthier Andreas Demetriou In the 1980s. Roth has had five of these "Sky" guitars made. To be able to emulate the high notes of a violin, all of the Sky Guitars have more frets than a typical electric guitar. The first Sky Guitar (used on the album Beyond the Astral Skies) had 30 frets. Later versions of the Sky Guitar overcame the problem of the higher register frets becoming too narrow by widening the frets by whole steps for the highest notes. In an April 2001 Guitar Player magazine interview Roth reported that the guitars are either fretless above the 30th fret, or have whole step fret spacing above the 27th fret, with 35 effective (half step) frets. All of the Sky Guitars with frets have scalloped fretboards. The Sky Guitar's pickups are custom 4-coil humbuckers, made by John Oram, with one guitar having an Oram pickup hidden under the 24th fret. For a time, Roth used a Framus Dragon amplifier, although currently he promotes Blackstar Artisan 100 & Series One 200 Watt heads. He has also used a stalk mounted Vibesware guitar resonator (sustainer) to introduce infinite sustain as on the instrumental Benediction on his Under A Dark Sky album. Dean Guitars produced 25 Custom Sky Guitars based on Roth's custom models with six and seven strings in 2011."

Roth has been playing scalloped fretboards since is days in Scorpions as stated in this segment from an April 2001 Guitar Player Magazine interview:

"There are five Sky guitars in existence, — three 6-strings and two 7-strings," Uli Jon Roth says. "The 7-strings have a low B and a range of over five-and-a-half octaves. When you have more than 30 frets, it gets very awkward to play in the upper register because the frets are so close together. So one of my guitars is actually fretless at the top. On another, I space the frets out in whole-tone increments. I can still get the half-steps, but I have to jump over to another string to get them. The fretboards are scalloped — which is something I never did to my Strats back in the Scorpions. It's still a hindrance for playing rhythm, but I definitely prefer a scalloped neck for single-note work — it gives me much more control over the notes."

In this video, at 3:08, you can see from the lighting that Gottardo is using a Scallop-fretted guitar.