Aliasing pixiv → invalid_tag
Link to alias
Reason:
A tag referring to another image pool is unreliable because the tag is often missed , plus it tells no additional information that the source URL can provide. Many images of various, but usually good, quality come from here, so I personally think it would be strange to allow blacklisting. There doesn't seem to be any notable culture (of English relevance) of pixiv to be notable beyond its image hosting, too.
Alternatively, if possible, sources from pixiv could implicate the tag pixiv.
Updated by Genjar