I assume those saying they've never heard of it are young (though it is funny how many posters "have never ever heard of" anything negative or critical towards the RF even if those things were extremely high profile, yet have heard every single negative tweet or obscure gossip blog post about Meghan).
The bullying allegation against Diana were absolutely all over the press at the time, and over the past year or so tons of newspapers and magazines have dug out old newspaper articles from the 1990s to compare the bullying allegations made against Diana with the same ones made about Meghan. Almost word for word the same.
Hell the tabloids even said the same thing about Kate, when she and Will were first married (or may have been when they first moved to Norfolk) there were tons of tabloids headlines speculating on the reasons so many Cambridge staff had left, quoting "anonymous insiders" as saying that Kate and Carole were nightmares to work for and ignored protocol, and claiming the reason so many staff left the Cambridge household was because of Kate's poor behaviour. The Daily Mail did a two-page spread on "the Cambridge staff exodus" accusing Kate and Carole of driving staff away, which was barely a word different from the same articles they later wrote about Meghan.
In reality, royal household staff move all the time because of the low pay and it's just the culture there, and of course a major change in household or location is going to result in staff changes. Royals change staff all the time, the Cambridges lost more staff that the Sussexes during the period the Sussexes were being slammed for losing staff. Just media manipulation.
The media decides who we're supposed to hate and who we're supposed to love, and acts accordingly.