Hashtag following has been merged into the upcoming version of Mastodon, and will be available on all servers at the next update.
It's already available on some servers such as https://mastodon.social and https://mastodon.online.
Elsewhere on the Fediverse, Friendica has already had hashtag following for years now, across all servers.
Whichever platform you're using, it works the same way:
1. Log in through the website
2. Search for a hashtag
3. Click the follow logo (or + sign) in the top right corner of the results
You will then see posts with that hashtag in your normal timeline even if you don't follow the account posting it. Note that it only shows posts published after the follow happened, so you may not see tagged posts in your timeline immediately.
This only works with posts visible to your server anyway, it won't pull posts in just on the basis of the tag. It's effectively a filter of the Federated timeline.
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.