Looking for recommendations for an open-source, peer-to-peer clipboard sync that works across iOS, Android, and Windows and Linux, with no central servers or accounts, best to have end-to-end encryption). Does anything like this already exist, or is this still an unsolved gap?

Blip is a good option. Not on Linux yet but they are making an app.

Edit: Not Blip they aren't open source anymore

If everyone submits feedback on how they want the app it will turn out how we want it. So some basics being Flatpak & AppImage support

https://blip.net/

This doesn't seem to be open source

Wait they changed it!! It used to be open source unless I imagined it somehow. Wait whaaaat