(this is a placeholder for discussion in far future)

CentralAuth's edge login and subresource autologin are two of the mechanisms it uses to try to log you into all of the wikis when you log in just once. They do not work in browsers that block third-party cookies, and the user experience of being automatically logged in is unlike any other SSO system I've ever seen, and probably unlike the SSO systems our users are familiar with.

For these reasons I wish that we would remove them at some point after third-party cookie blocking becomes common.

However, we have promised things like "Special:UserLogin now logs the user in to every unified wiki simultaneously" for a very long time, so this isn't a decision we can make lightly, and it would come as a surprise to users whose browsers continue to allow third-party cookies (or who allow them themselves).

It would be a lot of effort to document the trade-offs, so I'm not going to do that right now, but it should be done one day.