I think, I understand it now.

Thanks again for your time and very helpful explanation!

(As Picard user it is not obviously which tags came from which source and why they are treated differently)

It should work for plugins actually marking tags for deletion, but $keep is not one of those. $keep marks the tags you want to keep from MusicBrainz (not the tags you want to preserve in your files). It does not remove tags from the file, it is kind of a reverse $unset: With $unset you need to specify each tag you don't want to be filled from MusicBrainz, with $keep you do it the other way round and specify the tags you want to filled from MusicBrainz.

For explicitly removing tags from the file use $delete, that's its purpose.

Thank you very much for this patch!

I'm just curious:
Does this help also for plugins like $keep?
If I don't have acoustid_id in the list of tags to keep, will acoustid_id also be deleted through $keep or do I have to write the command $delete(acoustid_id) separately?

https://github.com/metabrainz/picard/pull/1064

Yes, deleting via the UI works (the entries gets red and after saving them they where removed).

BUT you know what:
After rebooting my test environment and therefore restarting Picard 2.1.0dev2, the above tags 

tracknumber
totaltracks
discnumber

becomes red automatically (as needed and expected)!

It seems, that my scripting changes for $delete(tag) doesn't get active immediately. Maybe a Picard restart is needed?

https://tickets.metabrainz.org/browse/PICARD-1435

The problem remains for the subject "acoustid_id".

I can remove it only manually (it doesn't get red, it disappears immediately).

And all the above entries are gone after saving the changes with CTRL-S (or Save-Icon)

Does it work if you delete those via the UI?

Thank you @outsidecontext 

Just for the record:
I can't delete this tags either with $delete:
tracknumber
totaltracks
discnumber

For other details like "track length" it is obvious that I can't delete it.
But it would be great to have control over all tags which we can manually edit in Picard (right mouse click, edit / remove) and like it works already for "genre" or "date".

Ok, then this is somehow related to the special handling of acoust IDs, I'll look into it.

If I try to

$delete(acoustid_id) 

in the scripting section, this has no effect.

As soon as I do a "Scan" I get the AcoustID and can't get rid of it anymore.

If I try to delete other tags like "genre" or "date" I see the red entries and in addition the red striked through value on the right side. It seems, it must be something special with "acoustid_id".

(Picard 2.1.0.dev2)

$unset cannot be used to remove existing tags from files, it will just unset the value from data loaded from MB. Essentially that means if the tag in question is already in the file it will be kept (unless clear existing tags. And since the acoust ID is not loaded from MB it essentially has no effect here.

For the use case of actually marking a tag for deletion Picard 2.1 introduces the $delete function. So $delete(acoustid_id) should actually work, but I have not yet tested it.