Why you think it is undocumented?

Have you seen comments in https://www.openstreetmap.org/changeset/136075427 ?

> Performing an undocumented mechanical edit (the text at https://wiki.openstreetmap.org/wiki/Mechanical_Edits/Mateusz_Konieczny_-_bot_account/fixing_malformed_shop_tags does not cover this case).

That is because it was removed in https://wiki.openstreetmap.org/w/index.php?title=Mechanical_Edits%2FMateusz_Konieczny_-_bot_account%2Ffixing_malformed_shop_tags&type=revision&diff=2527714&oldid=2525269 to document that it will not be done again.

There is "Additionally https://www.openstreetmap.org/changeset/136075427 resulted in stopping to change shop=fixme"

Also, shop=fixme was in https://lists.openstreetmap.org/pipermail/talk/2023-April/088185.html from thread linked on documentation page and in changeset comments here