Lemmy.World is the largest instance, and they preemptively defederated from hexbear.net a year ago, citing several examples of user comments that they wanted to protect their own userbase from.
Many other large instances have done similarly - another one is programming.dev (statement), although in their case if was merely to prevent one-sided conversations after hexbear.net defederated from them. The funny part of that story is how the admins took a vote, which indicated an unwillingness to defederate (27 to 19) but then did it anyway:-).
Anyway, many users of hexbear.net have made quite the reputation for themselves around the Fediverse, to the point where MANY instances felt the need to defederate from the entire instance (think: Truth Social but claiming to be leftist). And at this point, many users on it seem proud of that or at least consider it part of the cost of traversing the wider Fediverse using an account based on hexbear.net.
A summary of what happened between Blahaj.Zone and Hexbear.Net
Ada, an instance admin of Blahaj.Zone is extremely welcoming and inclusive - I have always actively looked forward to reading anything they say wherever/whenever I encounter such across the Fediverse, and have never been disappointed so far. They, like the instance overall, are the real deal(s).
Unfortunately, hexbear.net merely pretends to be thus, but like everything else they do it is a farce while their real goal is to be argumentative. They are actually quite open about this if you read past the initial prepared statements (or even read the official sidebar of the_dunk_tank) - on the instance they call it "dunking on" people, or "struggle sessions", and no matter how many users, mods, and even admins/devs they turn away as a result (as that vote post mentions), they cannot seem to help themselves, even outside of their echo chambers where that is allowed & actively encouraged. Even/especially when their own admins beg & plead - and yes, also explicitly command - their userbase to FUCKING STOP... they will not. And so far I haven't even mentioned the brigading... no matter how deeply I delve into this, there is always more.
Defederation is - and should be - always a last resort. That said, it is necessary sometimes, b/c as it has been said the only thing that can never be tolerated is intolerance - e.g. if cancer cells refuse to inhibit their own growth, then it must be cut out for the sake of the rest of the body to live. Also, as you see, it was actually hexbear.net that initiated the defederation from most of those places not the other way around - Lemmy.World did not wait to be offered the chance to be on the receiving end, but for programming.dev, Blahaj.Zone, and Lemm.ee (which was kicked off but later refederated, read also from hexbear) it was hexbear.net's choice, and the reverse defederation was only in friendliness to avoid dangling one-sided conversations where users on their platforms could talk to those on hexbear but the latter would not even know that there was a message awaiting their receipt, and subsequent response - as you are asking about here but in reverse.
Startrek.website and mander.xyz tend to have their own focus and not attract all of this drama, hence why someone from hexbear.net can see the post and also comment on it, but users on e.g. lemmy.world, programming.dev, or lemmy.blahaj.zone would not see that comment. The hexbear user is thus shouting into the void, with respect to those instances, though not to ours who can see both.
The sad fact (imho) is that new visitors to the Fediverse will never have any of this explained to them - instead, they have to do something like personally make the mistake of replying to a post in ChapoTrapHouse, found by sorting your feed by All rather than Subscribed, and then after WEEKS and WEEKS and WEEKS of batshit insane responses that do not end despite zero response back from said user, they finally will know what to expect from hexbear.net: overall (as a pattern) they enjoy argumentation, do not constrain themselves to that process being logical or the statements to be factual, but they sure do enjoy the "dunking on" process, so long as it is delivered from them to others, though obviously not in return. And from the other side, users of hexbear.net will notice how very few people "react" to their posts made in those communities they can see outside of their instance - essentially for them the Fediverse contains fewer users, from their perspective. And then from our side, we see the drama between their sides, more so than even they do themselves.