if you joined lemmy.world for example as a new user, you won't be able to see new comments made by users of beehaw.org. Only those who were made before defederation because they're still part of the copy of your instance.

Edit: Had it backwards, you can see their posts, but you're still affected by the defederation because you can't interact with them since they won't see your posts, regardless on which instance you make it.
And if you comment on a community that is actually hosted direclty on beehaw.org, no other user from any other instance will see your posts because beehaw won't publish comments from your instance to the other ones.

The admin of lemm.ee just wrote a long post about the consequences of the latest defederations of instances that are most likely spam bots

What does defederation mean for me as a lemm.ee user?
You will not be able to see any new posts or comments from defederated instances made on ANY instance.
You will still be able to see old ones that they made before defederation
Users from defederated instances will not be able to post or comment at all in communities hosted on lemm.ee