Technically yes. Due to the way Lemmy works, there are no Lemmy-wide terms of service.
HOWEVER
You would need to find an instance whose admins are willing to host that discussion. Otherwise you'd find yourself banned fairly swiftly as J. Random Admin wouldn't want to touch it with a barge pole. You would also find that instance eventually gets defederated.
No, they aren't blocked. You can verify this for yourself by visiting https://lemmy.world/instances and searching for the text "lemmy.ml" (left column means they're connected, right column means they're blocked, not appearing means the lemmy.world server doesn't know that lemmy.ml exists)
You can also visit https://lemmy.ml/instances and search for the text "lemmy.world" just to check the block isn't happening the other way around.
I agree with others that what's happening is likely to be an issue caused by the vast number of new users that are joining Lemmy over the past few weeks (myself included!) - give it time, things will settle down. :)
Better yet, forward every email you receive to your information security contact. ESPECIALLY emails that you receive from information security. Their email might have been hijacked. You can't be too careful.
One of the benefits of the fediverse is that there is no over-arching code of conduct, precisely because there's no way to enforce it. Each instance's owner decides on what they are willing - and not willing - to host on their own instance. Other instances can "defederate" from yours if they don't like what's hosted there, which means the two instances will stop talking to each other, as beehaw did with lemmy.world. But there is no way to globally ban an instance off the face of the internet.
Welcome! I second https://lemmyverse.net/communities/ - it's a great way to find migrating communities and also new ones that have sprung up organically!
Also, for future reference, please put the question in the title of your post (rule 1). This helps people know if they're gonna be able to help you before they even click to read more. :)
Cheers. Normally I prefer light mode because I find it easier to read, and keeps me awake more during the afternoon slump. But it's very nice to have the option.
Morning. I have one heck of a migraine today. Like a hangover, only I didn't drink anything last night.
This feels like a good time to remind people that Lemmy does have a dark mode. You can enable it by clicking on your name in the top-right, then Settings, then scrolling down to theme and choosing "darkly" or something like it.
Glad you were able to figure it out in the end. We're all still figuring things out, no worries. :)