Apps targeting Android SDK 30 published by F-Droid cannot yet be installed on Android 11. This affects at least FairEmail, Netguard, OpenVPN and a few others.

You might want to hold up upgrading your device to Android 11 for a few weeks if you rely on any of these apps.

As a developer, you might want to hold up upgrading targetSDK to 30 for a bit.

The issue is tracked here, including a workaround for getting your data back from FairEmail in case you already fell into this trap.

https://gitlab.com/fdroid/fdroidserver/-/issues/827