@MichalBryxi hi! I'm one of the authors of ActivityPub and the developer of tags.pub.

There is not a native ActivityPub way to follow a hashtag.

Following a hashtag in Mastodon only shows you everything with that tag that is shared by someone on your server, or someone that someone on your server follows.

The point of tags.pub is to follow hashtags globally.

If you don't want to participate, block away!