There were some casinos had such bug bounties but I cant really remember the names.
These days it is indeed rare to see casinos open a bug bounty publicly but I think most casinos has their own policy about bug bounty so if there is someone who find significant bug in the casino then he/she may contact them privately.
I believe it is how it works now, if you find a bug (serious one), better for you to contact them through the support and ask them if they have reward for anyone who find a serious bug.
Other possibility why it is lack of bug bounties now is that most casinos had tested their own platform well, perhaps they hired the best person to check the platform before the launch.
It makes them confident enough that there is no more bug in platform so they do not need to run bug bounties.
Yes, there were few. I remember one, exbet.io, they offered some users to test their site for possible bugs.
I believe they gave a lil bit amount to test their site and report whatever they find issues or bugs before the actual launching.
But not many new sites are doing this kind of effort. For some long-time casinos, they may have their own security team running such tests.
Because these large casinos with large number of players as well as bankroll amount, definitely, they have certain department taking care of these troubles or possible issues.