There are certain individuals who can't be mentioned on MN, because some posters are obsessed and lose all track of what's a reasonable or acceptable thing to say.
Look at Maddie McCann. We can discuss her now that it's been pretty much confirmed that the parents were not involved. For years any mention of Maddie would get a thread deleted, because there were - quite frankly - some total arseholes who would share hurtful and libellous 'theories' about her death. Some people are just incapable of seeing celebrities/public figures as real human beings. So because of the handful of posters who can't or refuse to control themselves, none of us were allowed to post about Maddie.
It's the same with Meghan. Most people discuss her in a reasoned way. But there's a minority of posters (and often they aren't Mumsnetters at all but people with no posts outside of Meghan threads, who spam dozens of different forums with anti-Meghan stuff, and who sockpuppet like mad and keep registering new accounts after being banned - these people are not MNers who just want to discuss Meghan!) who are completely fixated with hating her, who have completely demonised her to the point they can't even see her as human, who engage in overt racism and spill all kinds of vitriol and abuse.
Look how many threads about the current situation with the Sussexes have been derailed by posters harping on about online conspiracy theories (non-existent sex tape, Meghan being a former prostitute, Archie being a fake baby) or irrelevant stuff that happened years ago like how successful Meghan's acting career was, or why Meghan is EVIL for wanting scented candles/diffusers at her wedding even though Kate had the exact same thing. Almost every thread about the Sussexes gets derailed into an endless recitation of every single thing Meghan's done wrong from birth onward, every single tabloid lie or online conspiracy theory. Often with covert racism and nasty personal abuse to anyone with a different opinion. The haters even insist on gatecrashing the pro-Meghan threads, because they are so fixated on hating her and refuse to accept that anyone doesn't loathe her. It's honestly evangelical, like some kind of hate cult.
It's just the same as the posters who derail every thread about unsolved mysteries with conspiracy theories about the McCanns. MNHQ couldn't police the minority who couldn't behave so they banned it. MM is likely to go the same way if posters don't stop creating dozens of goady threads and just stick to the two long-running threads in the Royal section.