phospmph That's bad news. In their post in that reddit thread you linked, they wrote, it's "possible" they lost the keys. No further information.
But there should be a way to solve this. I think at least the Accrescent maintainer should be able to delete the old version or make it unavailable and the Easy Notes dev could run through the registration process with new signing keys again. Or something like that. But of course, that would have to be communicated.
They also wrote in that thread, the updated version should be available on F-Droid within 2-4 days. That was 3 months ago. As I understood updates on F-Droid (the main part of the store, not additional repositories) would be done quite automatically by scanning release tags on Github, catching the sources and producing the builds. So also unclear, why this did not happen within the last months.
Looking at the Github repos of Easy Notes and the developer's other project Athena shows, that there seemingly doesn't happen anything since January. Not in development and no comment or action in opened issues since then. (Except one short question by the dev in one issue by February.) There may be some unknown reasons preventing them from working on it.