F-Droid has high inclusion standards (not high enough IMO but apparently too high for many Android developers). If a project isn't in F-Droid and has no interest in being in F-Droid I consider it a red flag, but it's crucial to find out if an issue has been opened and what the project's response on that issue is. Sometimes it's just because the developer(s) haven't gotten around to it yet, but other times it's because there's a proprietary component that can't be easily removed.

For example on this app (2fas) the reason it's not on F-Droid is apparently because it uses Google cloud messaging (FCM) and there's no interest in developing a version without. https://github.com/twofas/2fas-android/issues/14

Unfortunately with the security FUD against F-Droid peddled in part by PrivacyGuides and other organizations (which Obtainium and its fanbase happily help spread) there is decreasing interest even in using, let alone developing for, this repository.