editline is intended to be API compatible, and has weakened the argument that linking implies work-derivation at least somewhat. As far as I know, nobody has ever been willing to test that theory in court since they'd lose so much political face in the process.

Sometime last year, I saw an article claiming that because of libedit's existence, we could all link against readline from whatever we wanted (or at least that this showed that the dynamic-linking-is-deriving was crap), but I can't find the article now.