If I understand correctly, users of other fediverse things like Mastodon can follow lemmy communities (and users?) but the reverse is not currently possible. Would it not make sense for lemmy users to be able to subscribe to a hashtag or user on a mastodon instance?
Subscribing to hashtags doesnt really make sense. Activitypub groups (which Lemmy uses for communities) work by following an actor, and it then sends all new items to the follower. On the contrary, hashtags are displayed simply by taking all locally known posts which contain that hashtag. There is no way to follow anyone to get all new items for that hashtag, and no way to have moderators or a description for the hashtag, like we have for communities. Its just a very simple and primitive implementation that doesnt work with the data structures that Lemmy uses.
To put some context, all I’m saying is just from a user perspective, I have no idea about the difficulty to implement any of the ideas I am mentioning.
Note that Friendica allows to follow hashtags, so I’m wondering whether a similar thing can be done in Lemmy.
This is true, and makes a hashtag more of a “de facto community”, different from the existing communities that are AP groups.
Maybe, rather than (or in addition to) their separate thing, it would make sense to have hashtags as a way to provide content to a given communities. Meaning that a community could subscribe to hashtags and automatically reshare the known posts carrying them.
Sure, following hashtags can be done locally on an instance, but its not possible over the activitypub protocol. So it only makes sense if you follow users, and then show their posts with a hashtag as a kind of local search.
The extra comment in that FAQ
seems to indicate that it is indeed not an actual follow in the AP sense. Note however that it does bring into my timeline posts from people which I don’t follow and which are on other instances.
So in Lemmy terms it seems that it is like searching the “All” timeline for the hashtag and including the resulting posts in the “subscribed” one. But of course the feature asked about by OP would involve that operation on a bigger “All” timeline that also includes toots.