I've been working on an open source trophy/achievement system. It consists of two pieces. The Gamerzilla plugin for Hubzilla allows you to show your achievements online. The LibGamerzilla library is for game developers and game launchers. The expectation is that a games would use the library to talk to the game launcher. They wouldn't need to worry about configuring and authentication. The game launcher would send updates to Hubzilla or if you didn't want to use Hubzilla, all the data would remain local.
I've recently got GameHub displaying achievements. If itch.io wants an achievement system where it doesn't have to host the data, this could be a good addition. I'd be happy to provide some assistance if needed.
Video is a little out of date as you can see with the GameHub support. GameHub has not been merged to the official branch yet.