The th-renaming-tiddler hook was added to allow plugins, to “catch” rename actions that have been activated through UI actions with the tm-rename-tiddler message.
The TW store itself does not know a rename. The TW store is immutable, so it can not be changed anymore, once written. That means if a tiddler needs to be renamed, the new tiddler has to be written and the old tiddler has to be deleted.
For this usecase, every action in JS is already too late. The warning has to be given in the UI, before the user clicks the OK or Save-edit button.
We do have 2 existing mechanisms the show such a warning.
The default core relink warning, which is related to the tm-relink-tiddler message
and the Invalid Character in Title warning
IMO that’s the only place where such a warning is actionable for users.
The save button triggers the tm-save-tiddler message.