I'd be happy today if I knew what to reset in user CSS for the 'everywhere' option. I have a 1200px res or so, so nothing super high-def (which is the problem case when you start looking at removing the max width). I don't know about other people.

I agree also that Wikidata could use it given the quantity of floating boxes they have and because we're not reading prose text there ever, in fact. Maybe this would be a reasonable [[mediawiki:timeless.css]] modification there? I know that mobile team doesn't want too much wiki-specific CSS, but I don't see that it needs to be a preference to support that. Would also want this for structured data anywhere I guess, so Commons file pages as well at this time. Maybe commons client-wiki file pages as well because I expect the structured data will be displayed there?

I'm not sure about proofreadpage. Are there other extensions you think could use it?

Category pages in general could benefit from removal of the max width. Other special pages like AllPages?

For now, basically just override this:

.ts-inner {
	max-width: 115em;
}

Well, if we have a user setting, it'd be pretty trivial to have it as a wiki setting, too, but even in the case of wikidata that's still an extension (or set thereof), which is also perhaps a bit saner to target in practice...