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?


Well…
there really is no tool for full bidirectional support. Probbably android (and ios) not letting random apps get the clipboard on the background.
on ios it can be achieved with shorcuts, not sure about android
Back in the day Tasker was the ultimate automation app. Don’t know how it has aged.