Someone also pointed me to this similar page: https://10friends.info/

Interesting to mention that Darius Kazemi maintains Hometown project, a fork of Mastodon that supports local-only posting among others. And Darius is now considering implementing the concept of Neighborhoods on top of that.

Now also on featured on Lobsters with a very positive comment : https://lobste.rs/s/rnsi0u/how_run_small_social_network_site_for_your