We removed the ability to unlink a character because of an order of operations / identity problem.
Unlinking a character effectively orphans it from its parent account, which is permanent if the server goes away due to a merger and can never be re-verified. This includes all of that characters forum posts, screenshots, comments, etc.
If they unlinked the character and then tried to set up a new one, people were finding that all of their past website activity had effectively become orphaned and unassociated with, well, them. And there was no way to undo it, because the original server was now gone.
A "transfer" mechanic to move all of one character's records to another would be far too much work to implement, plus be similarly difficult to verify, as people's character names could have changed as a result of the merger.