The URLs are a bit different with those systems. https://social.kernel.org/users/torvalds.rss works (and redirects to https://social.kernel.org/users/torvalds/feed.atom).

Thanks for pointing this out to me, I'm sure I'm not the only one who didn't know this. I recently wanted to import my own Akkoma RSS feed into my website, and I tried searching around the Akkoma documentation and didn't see any mentions of feeds.

Your URL is slightly off. If your feed reader supports auto-discovery (most do) you can just paste in https://social.kernel.org/torvalds (the HTML UI) and it will find the feed for you.