What do you mean? After android 7 or 8, the "install unknown apps" permission was per-app rather than being a global setting[1]. That means you can allow f-droid to install apps but not your weather app. The only difference is that third party apps can't install apps "silently" (you have to manually confirm each install/update through a system dialog).
[1] random image result: https://media.kasperskydaily.com/wp-content/uploads/sites/92...
Honestly I think there should be a distinction between normal permissions and permissions required to install other APKs.
AND I think it should be more cumbersome to install an APK that wants permission to install other APKs.
That's a huge difference. Try downloading and maintaining apps installed with F-Droid on a device where it doesn't have system privileges. It's a massive pain in the butt.
That's at most 30% of devices?
And will eventually be all/most devices.
I am on 7.1.1 and Allow Unknown Sources is still a global on/off toggle.
38.7% (8+ only) or 57.9% (if you include 7)