(Sorry if you get a notification about this comment twice, I first used the wrong comment box)

> Imho it doesn't make sense to "unset" this tag

Well, we can call it "not writing the tag" if you like.

> but software supporting ID3v24 is

YMMV, but in my small reality bubble, the basic stuff you need is supported quite well :)

> I fail to see how it hurts

It wastes space, it only works for one specific version of ID3 (not at all for other tag formats), it can not be used to decide whether you need to retag your files and it's largely the same as disabling the "Preserve timestamps of tagged files" option.

> but Picard is supposed to be the premier tagger, so if it is in the spec, then we should support it.

A write-only field is not what I'd consider "support".

> I can add an option

If others think we really need this tag, please do, but please ask a few more people (on IRC, the forums) about its default value (my vote is for not writing the tag by default, obviously :) )