Exchanges and casinos can have lifetime trading, withdrawal thresholds for different KYC levels.
If Shuffle use these for their platform and KYC requirements on their users, they have to write it clearly in their ToS. They can not arbitrarily pull KYC card anytime they want and it is only harmful for their reputation.
I didn't see accusation against Shuffle so assume they are good but to avoid damage on reputation, from now they can return to discuss internally with their Compliance team, lawyers and finalize their service terms on KYC, and add it to their ToS.
In general, almost everything has been explained in T&C even when creating an account, users are required to click agree on the T&C Box and information about KYC is available on point 4.1. (4.1.
We reserve the right, at any time, to ask you for KYC documentation ) although the reasons and withdrawal thresholds are not explained, this point can cover everything. If a user complains about this it would be better for him to mention the withdrawal amount so that we know about the threshold.
Apart from that I think casinos that use licenses probably use almost the same T&C so this kind of thing is nothing new, I suspect his last withdrawal around $2000 or more usually it requires KYC and most casinos on this forum also apply the same rules. As you said, if it is necessary, the compliance team should add details about the withdrawal threshold.