In case there is still some confusion, after the switch to the ICRC-2-based flow, a user or canister could still send ckBTC to a withdrawal account. This is a regular ledger transfer and will always be possible.
The problem is that the ckBTC minter will no longer consider these withdrawal accounts, so the funds will simply be stuck.
The proposed solution is to introduce a new endpoint on the ckBTC minter that allows any user to transfer funds back out of the user-specific withdrawal account.