I agree with you that "server-managed" E2EE wouldn't really be E2EE, and I agree about most proprietary platforms lacking the necessary transparency around this

I agree with you that "server-managed" E2EE wouldn't really be E2EE, and I agree about most proprietary platforms lacking the necessary transparency around this. From the XMPP perspective though, I want to clarify that ejabberd does not have "its own E2EE" and the E2EE that is used in modern XMPP apps (OMEMO) is client-managed and allows you to verify keys using e.g. a QR code. OTR's limitations are quite significant (lack of file sharing, group chats, offline messages, to name a few). I don't think that helps E2EE adoption. Unless someone picks up the OTRv4 work, but even that had excluded some of those items from its scope IIRC.