As a new user, I wonder the same.

Scrolling through the linked instances and noticed Lemmygrad was banned? Is it their politics or are they just annoying or smth.

The lemmy.ml instance federates with lemmygrad.ml, the collection of Marxist communities. It blocks lemmygrad.com, which currently redirects to the forum of choice for President Donald J. Trump. The latter does not seem to be hosted using lemmy and I think could not be federated with in any case? But presumably this was once the domain of a similarly-minded lemmy instance.

Some instances other than lemmy.ml do block lemmygrad.ml. Besides being a place for Marxist communities, the instance is also home to some very radical and very hostile users. I haven’t been around long enough to really know the situation for myself, but I have seen mentions of lemmygrad.ml communities engaging in brigading in the past.

For you, @[email protected] as a lemmy.ml user, lemmygrad.ml isn’t blocked, search for lemmygrad on https://lemmy.ml/instances to confirm. Other instances (like beehaw and sopuli) did defederate with lemmygrad. Lemmygrad is a topic-oriented instance (or at least an instance with a very strong cultural backdrop) of radical leftism. Not the mainstream liberal progressive left, but like Marx, Lennin, socialism, communism, radical anti-capitalism. Users on instances with cultures that lean in liberal progressive directions often clash strongly with users from this instance with a strong radical left culture.

This culture clash regularly causes flamewars and moderation shenanigans. As a result, some instances defederate with lemmygrad. But again, lemmy.ml is not such an instance and lemmygrad is not blocked for you.