How would you tell your users to take self-custody?
Just explain why you are halting custody service. "We don't want to be responsible for your funds, we don't want to be targets for regulation, we don't want to know anything about our customers, we believe in the mission of BTC to be peer-to-peer cash with no middlemen and we don't want to be your middleman, etc."
This should communicate the drawbacks of using third-party custody. Whether, they decide to send to their own keys or another bank is out of your control and you shouldn't care what they do with it. Absolving yourself of a custodial liability is enough. Communicate that holding their funds is actually a liability for you and maybe they'll second guess who they want to be liable for their funds going forward.
Thats Bluewallet-style exit. Lets say I dot want to exit....
But I guess you are right there should be element of scaring them. In a constructive way.
Lets say I don't want to exit....
Could require (or highly recommend) a 1-time course/quiz before they withdraw for the first time. Some kind of flow that teaches them about the drawbacks of third-party custody and how to self custody simply and safely.
it might be ineffective but it's a touchpoint. Maybe the 5th or 10th time they hear about self custody, they'll start to listen.
At the extreme, you could only allow withdraws to xpubs. Instead of entering an address, you require an xpub and generate unique address for each withdraw. I don't know any custodian that gives you an xpub so you can almost guarantee they withdraw to keys they control. Not the best UX tho...