On main sites, retagging is usually indeed too minor to be worth a notification.
On meta sites however, this is different.
I would like to see the following implemented: (i.e. not exactly the original request here)
- In case a moderator is adding or removing a moderator-only tag (aka "red tag") to/from a question, this will bypass the trivial edit check and always send notification to the OP.
- The above will apply only for meta sites. (Implied, as only meta sites got red tags.)
Worth to mention a comment I wrote four years ago:
That's considered trivial edit, same as changing one or two characters for example. It's the mod/dev responsibility to also post at least a comment saying "this has been implemented", ideally full "Nick style" answer with details.
The above is true, but many times a developer posts answer like "This will be fixed soon", then when it's actually fixed, only add the status-completed tag and at most edits their answer, neither of those trigger notification. Hence it's a good idea to force notification upon retagging in such case.