This feature has been released. It's still considered beta at this time. The docs explain how it works: https://docs.syncthing.net/users/untrusted.html

This wiki page is meant to collaboratively keep a list of on-going and out-standing tasks for the latest encryption proposal. For info on that proposal refer to the main PRs for code and docs:

Issues:
https://github.com/syncthing/syncthing/issues/62
https://github.com/syncthing/syncthing/issues/109
Original PR (most info): https://github.com/syncthing/syncthing/pull/6214
Up-to-date PR (most recent changes): https://github.com/syncthing/syncthing/pull/6727
Docs: https://github.com/syncthing/docs/pull/480
Forum discussion: https://forum.syncthing.net/t/latest-encrypted-devices-proposal-what-how/14164

Done

• Password consistency (https://github.com/syncthing/syncthing/pull/6673)
  Password token in cluster config to ensure all devices encrypting data for a single device use the same password.

• Configuring folder-device encryption passwords in web UI (https://github.com/syncthing/syncthing/pull/6691, depends on https://github.com/syncthing/syncthing/pull/6673)

• Mark a remote devices as untrusted.
  Ensures that no connection without encryption is established to that device. Sharing UI needs to represent that fact (modal is invalid if no password entered, visual clues to that effect, ...).
  https://github.com/syncthing/syncthing/pull/6787

• Testing with feature flag
  Testing (v1.15.0): https://forum.syncthing.net/t/testing-untrusted-encrypted-devices/16470 Early testing: https://forum.syncthing.net/t/early-testing-for-untrusted-encrypted-devices/15873 Feature flag: https://forum.syncthing.net/t/encryption-merging-scary-changes-feature-flags/15703
  https://github.com/syncthing/syncthing/pull/7055

Ongoing

Outstanding