Marking/unmarking entries has no more effect. Except for the changes in the bibtex source, nothing is currently shown. This also holds of quality, print status, priority, read status (as far as existing in 4.x).
Is that feature currently being revised, thus, things that I should see are just not implemented or is this a bug?
(I'm using Ubuntu 19.04 and JabRef 5.0-beta.342--2020-01-15--58709e7, Linux 5.3.0-26-generic amd64, Java 13.0.2)