Apparently, since the 0.18.0 upgrade, Lemmy doesn't have any outbound federation with non-Lemmy instances anymore.

Searching for communities, subscribing to communities and reading posts from communities on Lemmy 0.18.0 instances from at least Mastodon 4.1.0 and Hubzilla 8.4.2 no longer works. Doing the same with communities on the same instances running Lemmy 0.17.x from the same Mastodon or Hubzilla instances running the same versions still used to work.

Affected Lemmy instances include sh.itjust.works and lemmy.ca.

See also my bug report.