The official Syncthing app is no longer on F-Droid either. Syncthing-Fork is and will continue to be supported.

The official client has, but Syncthing-Fork is still being developed.

What we would need on the long run is simply replace email with a common standard

That would be ideal, but realistically, if email ever goes away, it would be replaced with a proprietary locked down ecosystem. Likely a messenger app. Link a WhatsApp or Facebook account and you will get messages and notifications through that. I just do not see current tech companies supporting a new open standard for communication.

Despite all of emails flaws, it is one of the few remaining universal forms interoperable communication with little vendor lock-in. It would be great to have something more modern, but not at the expense of openness and interoperability which is likely what would replace it at the current time.