@noam (I'm on friendica.world.)

It's an add-on:
Settings → Addons → Content Filter (Not Safe For Work-tag and more)

This addon searches for specified words/text in posts and collapses them. It can be used to filter content tagged with for instance /.../ that may be deemed inappropriate at certain times or places, such as being at work. It is also useful for hiding irrelevant or annoying content from direct view.

→ check "Enable content filter", add stuff in the "Comma separated list of keywords"

Both words and hashtags work.