Is this even possible and/or worth the time and money put into retracing transactions? Even if they (in general) agree to not receive transfers from non-compliant exchanges, how much effort will they put into it.
They just hire a company like CipherTrace to do it. I don't think it'll cost them much beyond what they're already paying for.
They already analyze customer transactions -- both in and out -- to identify transfers to/from casinos and sportsbooks, and they terminate accounts over it. This is to comply with the Wire Act and UIGEA. They are prohibited from transferring money to/from gambling businesses on behalf of customers.
I guess we need to wait and see what national regulations look like before making any assumptions.
What will happen if somebody sends money to a third party first and then to the exchange, or sends money from a casino, or from a mixer?
It's going to be one of those laws people agree to obey and then they don't like speed limits and no parking signs.
It's becoming increasingly common for exchanges to hold funds hostage and demand proof showing the source of funds.
To use Gemini as an example again, it's against their terms to send them coins from a mixer or a casino. They'll terminate your account over it. I hope they send your money back too.