The only true precedent we have here is the public Collection server run
by PediaPress, which is a wonderful service used by several hundreds
wikis for years. It ensures a comparatively smooth installation of the
Collection extension even for the most niche formats.
https://www.mediawiki.org/wiki/Extension:Collection#Generating_PDFs.2C_Open…
One could perhaps also consider DNSBL services by SpamHaus and others.
https://www.mediawiki.org/wiki/Manual:Combating_spam#DNSBL But I don't
know how many are using them and they're not really effective, so we
don't really have a service comparable to Akismet. The "Antispam"
extension claims to be one, but at the moment it's used by 3 wikis only.
Nemo