f-droid defines it as

Non-Free Network Services

This Anti-Feature is applied to apps that promote or depend entirely on a Non-Free network service which is impossible, or not easy to replace. Replacement requires changes to the app or service. This antifeature would not apply, if there is a simple configuration option that allows pointing the app to a running instance of an alternative, publicly available, self-hostable, free software server solution.

Here’s the list of apps with Non-Free Network Services.

https://f-droid.org/en/docs/Anti-Features/#NonFreeNet