Edit: this seems to be due to (removed for fdroid) google integration. Feels strange... Author removes a Google library for fdroid release but is still being punished be the antifeature.
It contains a google library, but can't find what exact library they are referring to, as the link they've provided is dead.
EDIT: Found it in an old commit from around that time the comment was made https://github.com/massivemadness/Squircle-CE/blob/1818d3904...
A googleplay "appupdate" library, I don't have knowledge about App development, but I don't see how this library is needed in the fdroid version?
> In our experience, where the upstream developer includes Non-Free libraries, sooner or later they will include more Non-Free libraries, or other Anti-Features. Frequently they become impossible to maintain/update in F-Droid.