Along with the most recent v5.6 release of K-9 Mail, the change log has moved. The old change log as referenced in F-Droid has no mention of 5.6, and as noted on the wiki page, is now for old releases only. The current release notes page is: https://github.com/k9mail/k-9/releases
As it stands, it is difficult to verify that version 5.6 is even a valid release with the links as given on F-Droid.