That doesnt appear to be true, the restriction seems to be on apps being installed from file managers, web browsers, messaging, etc.
F-droid and the like are not part of that list.
This still isn't good, but it doesnt stop you from having F-droid manage your messaging apps it would seem.
Edit: If you're down voting because you think its using the same method as a file manager as the user that replied to me, this is incorrect. This is also an issue going back several versions.
F-Droid uses a session installer method for 3rd party app stores, it does not use the same method as a file manager.
For an article about a similar issue brought up by similar restrictions in previous updates, you can refer to this article:
https://www.androidauthority.com/android-15-restricted-settings-sideloading-3481098/
You can also refer to this thread in the F-Droid forums which covers this as well, from 2 1/2 years ago:
https://forum.f-droid.org/t/sideloading-restrictions-or-removal-in-future-how-it-effects-fdroid/21089/10
Which also includes a merged discussion from the last time this came up 9 months ago.
F-Droid has been using the session installer method for quite some time.