I'm already starting to get pretty tired of people in the fediverse saying shit like this:
What this means to you is when a user within one instance (e.g. Beehaw) that’s chosen to defederate with another (e.g. lemmy.world), they can no longer interact with content on another instance, and vice versa. Other instances can still see the content of both servers as though nothing has happened.
A user is not limited to how many instances they can join (technically at least - some instance have more stringent requirements for joining than others do)
A user can interact with Lemmy content without being a user of any Lemmy instance - e.g. Mastodon (UI for doing so is limited, but it is still possible.)
Considering the above, it is important to understand just how much autonomy we, as users have. For example, as the larger instances are flooded with users and their respective admins and mods try to keep up, many, smaller instances not only thrive, but emerge, regularly (and even single user instances - I have one for just myself!) The act of defederation does not serve to lock individual users out of anything as there are multiple avenues to constantly maintain access to, if you want it, the entirety of the unfiltered fediverse.
Having "multiple avenues to maintain access to the unfiltered fediverse, if you want it" is the most nightmare user experience sentence I can possibly imagine.
A user does not want multiple avenues to maintain access to the unfiltered fediverse with it being unclear when their comments will be shadow banned and not. They want to be able to see a post and go in and comment on it.
Federation is not a feature, it's an implementation detail.