Sideloading has always been allowed, and that new "advanced flow" is something entirely decided by the OS so I don't think it has or will have any effect with GrapheneOS use.

It's actually recommended to get least of your apps as possible via the play store and use 3rd party stores if you are going for maximum privacy or de-googling.

I’m in the process of switching from an iPhone to Graphene on a pixel 10 Pro.

I can load apps from the web (disabled by default iirc), the local file system, a sandboxed instance of the Play store, Obtainium, f-droid, wherever.

Took some getting used to, but it’s nice feeling like I actually control and can choose how my phone works.