I don't know for most of casinos, but casinos that I use support TON: BC.game, Rainbet (only deposits, but no withdrawals) and Whale.io. I prefer TON as I have some funds in TON and withdrawals are pretty cheap.
It is possible that because TON has little demand from users that Cybet has not been able to add TON to their deposits, especially as @Questat said there are about 10 coins available there, maybe it is the top altcoin while TON is position 26 CMC so it cannot be added - although what I said is not necessarily true.
If Cybet adds more altcoins and say 20+ coins available for deposit then I'm sure TON will be on that list.
), i see its still quite new just registered last "Joined July 2025"... so its understandable that they only have few coins being added yet, but eventually they will increase it for sure now that a user had requested it.