It's worth noting that there's some cases where temp users having preference-setting completely disallowed is going to give them a non-default experience, because there's places where we've left old defaults in place for existing users and set new "defaults" by overriding them for newly created users. See e.g. Echo, which changes what notifications are sent in this way.

(Which is all fine. We should just be aware that it might cause confusion as someone moves down the anon->temp->registered pipeline.)

For the Wikimedia Incubator, it would be very useful for temporary users (and indeed anonymous users, but that's probably not gonna be possible) to be able set a non-default user interface language and a non-default test wiki preference. The interface language thing probably applies to other multilingual wikis as well. Maybe a solution could be keeping a whitelist of settings that can be changed by temporary users?

@jhsoby My comment here discusses some points relevant to this: T332414#9041449

There isn't a mechanism right now to allow preferences to be saved for temp users, but we're open to working out how we can do this in the future. Perhaps via one of the solutions mentioned in that comment, or some sort of allow list.

Tagging for @Niharika to follow.