Currently the option to add a signature (introduced by T53154) is allowed only in the User: and Pomoc: ("help") namespace. We need to have it enabled in other namespaces as well. The reason is there are pages requiring users to sign up in other namespaces as well, for instance pages of various wiki workshops (example), wikiprojects and whatnot, not only on talk pages.
We held a discussion and several people noticed lack of the signature functionality as a problem (though there was no formal voting, I doubt there needs to be one for such a cosmetic change).