As much as any casino or gambling platform asks for KYC in my opinion it is best to do it before depositing. Because a big win in the casino can happen at any time and at that time if someone is prevented from withdrawing money due to KYC complications, it will definitely cause human pain to the gambler. KYC is must for conducting uninterrupted gambling. However, if a gambler consciously doesn't want to do that then he will be responsible for any unwanted incident.
Usually before depositing with a new site I try to get to know about the site. If I can trust there then I make my deposit and if there is confusion then of course i have to read the whole terms and conditions. I'm not too worried about doing KYC for a reliable site. Who have already gained a good reputation in the market will never cheat their clients. I would definitely take the decision to gamble with utmost caution in the case of new sites.
This a hoax information. Certainly, a scammy casino that has no intention to let you withdraw a max win will still find a way to hold your funds even if you complete the KYC before you hit the max win.
Legit casino will release your win after you complete the KYC and user that usually spreading false information about their funds being hold after a big win are those user that playing from restricted country which their ID will obviously have a problem. But if you are non restricted players then there’s nothing to worry on completing KYC.