I think I've seen something somewhere saying that the prevailing sentiment
is that obsolete identifiers which are just redirects to a new identifier
should be removed.
There's also the case of sites like MusicBrainz which keep the
non-canonical IDs without redirecting to the canonical ID, but will tell
you which ID is preferred, e.g. Fritz Kreisler
where all the tabs for the second two pages actually point to the first,
Is there an established policy for either the redirect or non-redirect case?
I'd argue that even the obsolete identifiers are useful for inbound
resolution and reconciliation. Aggressively pruning them just makes more
work for people, because they must resolve the identifier that they have in
hand to its canonical form (probably by hitting the issuing authority)
before using it for Wikidata lookups.
What do others think?