On the web interface there is a block community button on the sidebar. This functionality is currently missing and I think will be important for newcomers who are using All to discover content. Similarly a subscribe from the feed could also help people build or rebuild a list of communities.
Tracking: https://github.com/buresdv/Mlem/issues/89
Coincidentally, in my large API sweep today, I have written the API request types neccecary to achieve this :)
After this big PR is merged, UI/UX developers could implement this on the front end, track here that big PR progress: https://github.com/buresdv/Mlem/issues/52