Interesting. How would you use it?

Let's assume the user wants to see all notes from their follows. The problem right now is (as some people have already commented in Will's note) the user sometimes scrolls for a few minutes, then puts the phone away and starts scrolling again in let's say 15 minutes. What happens when you come back is there are some new notes, but after scrolling through those, you need to scroll through the "already seen" posts from the previous session very quickly to be able to see notes you had missed previously. If you repeat this many times a day then the problem scales to a point where the user gives up and accepts missing a lot of notes that day. Having a boolean read state for notes fixes this problem. There could be a toggle or an eye icon for toggling already seen posts as hidden/visible. Some RSS readers have this functionality.

Could even optimize this by saving ranges of timestamps instead of individual read states 🤔

True. That would even enable toggling some ranges as hidden and some visible. A more fine-grained toggle. For example "Toggle the notes in the created_at range of {this morning} as visible". Not sure if people would use it though. The main point is to be able to hide already seen notes from the follows feed.

hum... an "unread feed" feels more interesting than a "move to the newest read event +1" button in the regular feed. Maybe we can do a toggle to enable and disable read posts so the feed can be always empty for those using the app in that way.

An "unread feed" would solve the scrolling problem as well 👍 or the toggle to hide read posts could be in settings, not in the feed itself.