Latest version is 6.0 while only 5.3.9 is available via F-Droid. The developer forgot to push the latest version of the App to GitHub but has done so a while back:Version 6.0 on Play Store? · Issue #115 · tom-anders/Easy_xkcd · GitHub
Versions
We don’t have the current version of this app. (Check mode: Tags ^release) (Auto-update mode: None)The current (recommended) version is 6.0 (version code 139).
Easy Xkcd | F-Droid - Free and Open Source Android App Repository