Ok, suddenly the casino you're playing asks you for proof of income to be able to withdraw your winnings and to keep your account in good standing, your bankroll every month is $500 and you provide proof of salary that you are making $500 net every month, will they accept that proof will they accept that you are putting all your salary for gambling, will this violate the principle of gamble for fun, because you are working to gamble based on your proof income.
For transparency, it will be fine if you let us know the site name and maybe some evidence as regards to this, so other people will take note of site.
To add to this, Why on the first place they didn't ask you to provide this during deposit or registration?, and now after you have made some winnings and trying to withdraw your winnings, they now requested you should provide them your proof of income.
This Is fucked, Exept you have deposited huge amount of money into your account that looks suspicious, but in the case you got the winings from the platform, I see no reason why they should ask for this when they can check from their end and see you transaction history with them.