www.wikisource.org redirects to wikisource.org (oldwikisource). This is the opposite of how all the other second-level-domain sites work (mediawiki.org redirects to www.mediawiki.org, wikidata.org redirects to www.wikidata.org, wikifunctions.org redirects to www.wikifunctions.org) or even how the portals work (e.g. wikipedia.org redirects to www.wikipedia.org). It also complicates cookie handling somewhat (and has caused bugs like T351685: I keep getting logged out on Wikisource in the past). Probably would make more sense to do it the other way around?

(See T64717: Move oldwikisource on www.wikisource.org to mul.wikisource.org for a different proposal.)