RL;DR: If you are a user of Android 7~10, use Legacy; If not, use Modern. Besides, Fdroid is exactly identical with apk on Fdroid;
ignore it.
Currently, we have three channel (Stable Channel, Preview Channel, and Fdroid Channel that is identical on F-droid),
and two distinct Variants:
ModernLegacy
We make this distinction mostly for bypassing Scope Storage for Android 10.
It targets on the latest SDK, and declares hasFragileUserData true (supporting uninstalling without removing data).
Target on SDK 28 (Android 9), to bypass Scope Storage introduced in Android 10
(especially helpful for Android 10 users since Scope Storage could not be escaped in this version).
Also, it declares hasFragileUserData false, to solve the problem of difficulty uninstalling1,
due to installer(uninstaller) crashing on Android 10 with physical SD card.
See also Development Guild $ Build Variants
Starting in 2027 worldwide (or September 2026 for some countries), Android applications from developers without verification centrally through Google Play or the Android Developer Console, may no longer be installed easily on certified Android devices.
In simple terms, APKs signed by unregistered signing keys, where developers refuse to upload their personal identification (governmental ID etc.) and their app information (package names and signing key fingerprints etc.) to Google, may not install easily on devices with Google Mobile Services (GMS) where Play Protect is enforced.
Apps distributed outside Google Play, just like Phonograph Plus, are severely affected. Although an “advanced flow” may exist, the " sideloading" would be much more difficult for most users. And Google is determined without any sign of regret for now.
Details and updates can be found here (offical) and here (3rd-party).
As a response to this policy, after April 1st 2027 (or August 10th 2027), APK artifacts will no longer be provided in releases but only source code. Please compile and build Phonograph Plus yourself.
If Android becomes further closed and restricted, Phonograph Plus may be really and eventually discontinued in 2028, depending on Google's policies next.