I would not oppose casinos to give some proof of liquidity or reserves, specially those who are not so well established (those are more vulnerable), one is never 100% sure the management of the casinos decided to relocate part their reserves on other places to get passive income or something like it.
Anyways, I doubt mos the casinos around here will suffer from this market. The World Cup is here and it is an excellent chance for them to attract more active users.
I think all casinos convert cryptocurrencies into stablecoins or fiat immediately after deposit. At least those whose base currency is USD or EUR or some other fiat currency. Those whose primary currency is Bitcoin should keep their reserves also in Bitcoin so that the exchange rate change does not affect their business either. 1 BTC = 1 BTC.
Judging the way some casinos offer fairly quick withdrawal in BTC, I would assume they keep a good percentage of money in BTC.