Install not sideload.

We heard from developers who were concerned about the barrier to entry when building apps intended only for a small group, like family or friends. We are using your input to shape a dedicated account type for students and hobbyists. This will allow you to distribute your creations to a limited number of devices without going through the full verification requirements.

we are building a new advanced flow that allows experienced users to accept the risks of installing software that isn't verified.

Today, we’re excited to start inviting developers to the early access for developer verification in Android Developer Console for developers that distribute exclusively outside of Play.

Does this mean F-Droid, IzzyOnDroid, Guardian Project, etc, are safe for now? The messaging is unclear.

What about other android based OS like Graphene, etc? Will they be affected?

They are just implying they will add a shit ton of friction to fdroid but not make it impossible