Developers who distribute Android apps on other app stores are not strictly required to undergo verification and thus can remain anonymous, but if they choose not to, then later this year (when the enforcement of verification goes active) their apps can only be installed on certified Android devices via ADB and/or the new advanced flow.

Thus, you can still install unregistered apps if they're distributed via F-Droid or other sources, but to do so, you will need to use ADB and/or go through the new advanced flow. And remember, the new advanced flow is a one-time process - once you go through with it, you can allow your device to install unregistered apps indefinitely!