Consider for instance https://musicbrainz.org/search?query="public+domain"&type=annotation&method=indexed
My trigger for this question is 3 releases:
all having recordings marked expressly ‘public domain’, but I see no way of notating this other that by annotation, and find no guideline. There is no [public domain] special purpose label for ℗ relationship, and would ‘public domain’ be a relationship itself, it should pertain to an [all as none] special purpose artist (that should already have been needed → created and present, which it is not) or carry an exceptional 1-way of entity:recording—relationship:publicdomain[—entity:none] (which seems a technical dead end).
What is the preferred way of notating an entity being in ‘public domain’, and are we happy as is?