Substack Notes is open. I have an account that I started a long time ago when I was trying out their newsletter publishing system. I ended up writing my own, because I didn't want to be locked in to using their editor. I like that I can edit my posts. Is there a character limit? I don't know. Can posts have titles? Where is the RSS feed for my posts? Does it support ActivityPub? Is there an API? The user interface is remarkably close to Twitter. More here.