Most of these are on Reddit, but there may be a few that I thought could be specific, but not as niche as some of the ones on reddit. (like no community just for one game)
Van conversion/van life
Birding
Camping and hiking
AmITheAsshole
Each state in the US
Longevity
Kindlescribe/e-ink
RestlessLeg (or a chronic condition community)
Woodworking (bigger builds)
Carving (smaller builds, not furniture or big stuff)