Wow, I can't believe they took it down and it won't return. Just poor and screams that they did it purely on percentage purposes. I managed to give it a good 2-3 hour bash last night and went up £100 on 20p spins but eventually lost £80 by the end of the session so even 99% could give you a rollercoaster ride.
I guess if you were a higher stake player and it behaved with percentage, you could bump it closer to 100% with the weekend booster thrown in and that's just too much for VS in terms of margins for them.
99% is a high RTP for a slot but in the case of Book Of 99 it's:
1) A single slot out of many that VS have on their books
2) Still runs with a house edge, so over time the casino will make money on it
I play mainly at Unibet.com these days and they've got the 98% slots selection, which it's nice to have available but it's only ten slots and with the best will in the world no one's going to keep plugging away at the same ten slots forever.
Looking through my 'last played' list on
Unibet, which keeps a record of the last 15 slots I played, only one of the 98% games are even on the list. (Twin Spin, which I do have a very soft spot for, but that's the case even in its normal 96.7% RTP version, so the fact it's the 98% version isn't even what motivated me to play it.)
I think it's a dick move for VS to remove Book Of 99 on account of its high RTP, maybe a focused enough player could tip it over 100% long term with the perks (such as they are at VS these days), but even there the casino always retains the 'we reserve the right to close your account' in the T&Cs if they're not happy about that. (Or they could do one of their famous SoWs, which seems to have the same effect

)
(And the volatility of slots is so high that the 'advantage player' would still be taking a serious risk with their money, on a game that, remember, still has a solid house edge. I kicked Book Of 99 around a fair bit in demo play on the link dunover posted and it's a spicy bugger, and it will chew down a balance if you catch it wrong.)
As players we all know that, ultimately, our function in the relationship is to hand over our money to the casino, but for VS to be so blatantly 'FU' about it speaks volumes IMO, but then, that's why I stopped playing there early on when they started nerfing their RTPs.