WhatsApp wasn't the first to open up but that makes sense given it has a few billion active users and an open E2EE encryption protocol is really difficult to implement.
It's rather easy. Especially if you deal with 1-to-1 chats. S/MIME exists since 1995 and it's not even first E2EE protocol. Like PGP. Double Ratchet E2EE proto that is used in signal, wa and matrix was created in 2013 for Signal. Although Olm(Matrix) uses sligtly different cryptoprimitives from Signal, WA and OMEMO proto.