Webmail is perfectly possible, if clunky,without JS. That's how they used to work. IRC and IM require real-time updates of some sort, which is a reasonable use for JS. A Twitter clone shouldn't require JS just to render a page.
You also can run a Mastodon application server side and serve server-rendered pages. I'm not aware of anyone having done that yet, but, you know, be the change you want to see. It's probably got privacy implications.
