@blindscribe @simonlindgren @WeirdWriter
Mastodon.social does have hashtag following.
You can do it by logging in through the website, then search for the hashtag you want to follow (click through to it, don't worry if it says 0 results), then look in the top right corner of the hashtag's column. There will be a little icon with a + on it, click that. (I've attached a picture showing it.)
When you follow a hashtag it means public posts with that tag will appear in your timeline.