I've had my first opportunity to see how this works first hand now actually with beehaw defederating with us and it seems to confirm this, except I can't seem to see new posts added which answers the question for definite lol.

So when on my lemmy.world account I can no longer see new posts from communities I've subscribed to on beehaw, but I can still see the posts from before defederating if I go to them directly. This is true at least using the jerboa app, maybe it's different on browser but I'm a phone user so I don't use the browser lol. I had to switch to my feddit.uk account to see new beehaw posts on my subscribed communities to confirm it wasn't just a slow community.

Hopefully it becomes easier to subscribe to communities on the app sooner rather than later so I can actually have the energy to join these communities on multiple accounts so if more defederation happens I can still just read them haha