Github repo is public now, in case something like that happens in future or if anyone is willing to contribute. It's kinda old so apologies in advance.
https://github.com/MilesVII/621watched

list said:
Another thing to add to known issues;

If a tag is aliased after you start watching it you can no longer tell from the tags on the image page which tag caused it to be included in the watch list.

Example you were subscribed to night_time but it's been aliased to night it shows night_time in search and in the extension but it shows night on the page so the extension can't recognize it to mark it as a subscribed tag.

I had this concern when making it, but didn't come up with a way to track and reflect these changes easily.