You can filter any Mastodon account's public posts by hashtag using the following website address:

(mastodon profile's web address)/tagged/(hashtag without the #)

So, for example, to see all my public posts with the hashtag "Mastodon" the address would be:

mstdn.social/@feditips/tagged/…

As many have pointed out in comments, you can also add .rss to the end of the address to create an RSS feed.

#Mastodon

Fedi.Tips (@feditips@mstdn.social)

3.1K Posts, 568 Following, 163K Followers · Posting hints and tips about Mastodon and the Fediverse 🌍 There are lots of answers to common questions about Mastodon and the Fediverse on the website at https://fedi.

^{Mastodon 🐘}