In most jurisdictions this project wouldn’t have any problem, when it’s a “clean room” implementation - meaning they figured out by themselves how the server works.
Several jurisdictions even allow reverse engineering and such to ensure compatibility / supporting an product that hasn’t any official support anymore.
Others simply don’t care.
It’s all in the details, but they might not get sued that easily.
Unless you never thought of, implemented, regularly did and regularly tested your backup of the database. Or… try to use it on more than one device - maybe even at the same time.
That’s the main problem with KeePass. It’s nice to have it offline, fully under your control and out of the cloud, but that comes with some responsibilities on your end. And now think of how the average user solves this. If you’re tech savvy enough, KeePass is great!