On Lemmy, it’s shown. On Mastodon, instances are given the ability to hide their blocked instances which IMO hinders people’s ability to join. A lot of the drama on the fediverse with regards to blocking is from Mastodon and similar instances.
People claim the instance you join doesn’t matter because “everything is federated!” but it kind of does. It strongly affects the people who will see your posts and even who you can follow, and if you don’t know what an instance is blocking it creates an air of uncertainty. I’ve seen instances get blocked for stupid at best and downright malicious at worst reasons, so the 4chan post isn’t inherently wrong.