Not sure where/how the n8n site actually sets this, but if you open developer tools F12, go to the Application tab and look at Storage → Local Storage, you can right click and edit the N8N_TEMPLATE_DESTINATIONS. Then reload the page and you should have your edited list as choices.
LOL, it works. My guess is that the email-installations association is done somewhere on n8n.io’s backend and not in my browser, but that is yet to be proven.
I’m hesitant about marking this response as “Solution” since it’s a (successful and efficient) bypass rather than a long term solution.
I’ll circle back to this in a few weeks if this remains stable.
When you click the Templates link from the n8n editor UI, your instance URL gets added to the list. I’m ashamed that this didn’t occur to me before. It’s SO obvious. Where else could it be!?
Also, if you want an easier way to edit the list (to remove the old entries), you can try out the UserScript I wrote to add a button for a pop-up editor.
Before you ask… yes, this is something I did just for fun, to knock the dust off some UI dev skills, and no, it totally isn’t necessary.