So I’m still learning all of this too, but my understanding is this:
Your accounts on each instance get you access to whatever that instance is federated with. My mind goes to bundling, almost – e.g., if you have a Hulu subscription, you can access Showtime through it as well.
When the instances defederate, it’s like losing part of a bundle. Nothing stops you from making an account on that instance, though, while retaining your accounts on other instances.
So, in how it plays out: I have my account on reddthat.com. When I scroll through my feed, I get content from communities hosted on reddthat and from communities hosted on instances that are federated with reddthat.
But suppose I want to see content from communities on instances that aren’t federated. All I have to do is switch over to another account on an instance that is federated or to an account on that instance specifically. I still have both of my accounts and all I need to do is switch.
The talk about shadowbanning just means that if you’re trying to access content from an account not in federation, you won’t be able to comment or anything like that.
Again, I’m still learning all of this stuff, so this may not be right and I welcome any and all corrections of anything I’ve said that isn’t accurate.