[j1elo]

From your first link, it seems the decision to use a different new domain stems from difficulties getting the server's HSTS policy right, and it even seems they had a similar issue in the past with having the store as a subdomain [1].

If that's true, for a use case as functionally basic as having a store and a social instance in their respective subdomains, it looks to me like a complete failure of HSTS, a case of technology causing problems that shouldn't exist to begin with.

[1]: https://phabricator.wikimedia.org/T337586#8920625

[marksomnian]

It's not linked there (or on any Wikitech pages I can find), but I can imagine there's a secondary concern of *.wikimedia.org cookies getting sent to third parties - e.g. Stack Overflow has separate second-level domains (stackoverflow.email/stackoverflow.blog) for their 3rd-party-hosted email service and blog for exactly this reason (cf. https://nickcraver.com/blog/2017/05/22/https-on-stack-overfl...)