There's a book that the hosts on Buffering the Vampire Slayer have referenced a few times that might be interesting if you haven't read it already. Amazon link here
I'm trying to keep a balanced view on this, because I have seen some radically critical things which are not unwarranted (in fact, my real tendency is to agree with them) but at the same time we can't erase the good things he's done, either directly as a writer or indirectly through creating these groundbreaking shows which had so much talent gravitate towards them and so much real impact in pop culture and in the communities created around them.
The truth is that he himself knew that he could not really be hailed as a feminist, back in his heyday. If you see how he reacts to such praise it's exactly like that meme from The Office not because he's humble, but because he knows he's not someone who has lived and operated in a way that's guided by feminist principles (Charisma Carpenter recently opened up more on her issues on Angel and it turns out they were even worse than what we thought). I have only seen excerpts from the Wonder Woman script, but they are telling of its far-from-feminist, male gaze style. So the recent change of attitude against him by people who used to think of him as a feminist writer absolutely makes sense.
And at the same time, I don't see anything wrong with revisiting and examining our 90s nostalgia: of course we see those shows as products of their times but there's nothing stopping us from examining them from the current lense at the same time. For me, critically examining these issues doesn't damage my relationship with my favourite shows, I love them for what they are, while acknowledging their flaws, some of which are a product of their times and some of which are cases of clever educated creators who *should* have done/known better.
As for the diversity issue, I'll defer to this very recent Buffering podcast episode because there's nothing I could say that they haven't said it better and more thoroughly (plus this whole book of essays, which again doesn't attack Whedon's creations but examines them from all angles as it should) .
People were discussing his problematic elements when Buffy was on air. There's a whole book of academic essays on just Whedon's approach to race that came out 7 years ago.
And I recall my personal first introduction to a Joss critique, was reading an essay lambasting the portrayal of Inana in Firefly, by a sex worker. And that was online not soon after the show left the air, and the mystique over the show's short run was growing rapidly -- it was not "popular" at the time to critique Joss, seen as a victim, yet there were early voices, and more than one, doing it.
Doing it not just for the clicks, but -- in many cases I found in those years -- doing it because they wanted to fall in love with Joss' works, too -- but the way he and his writig teams would shape something, would set them off. Would cause them to buck the tread, and push back against the very narrative you imply was 100% celebrated. It was not, and to erase those early voices is to deny them their rightness, now, in hindsight.
Was Buffy groundbreaking? Sure. But Star Trek was also groundbreaking, in so many ways, as a show very dear to my heart. Yet it was also so frustrating an experience, that Nichelle Nichols nearly left the show, something we didn't know about for years after the end of the Original Series, but if you rewatch, her lack of character development, or even plot beats, is really obvious. And yes, that colors my love of that show; it doesn't take it away, but it does make it richer, and that richness goes in many directions, some negative.
Our relationship to movies -- media, in general -- does not have to be static. Listening to, say, Roger Ebert's commentary on CASABLANCA makes me love that movie even more, learning more about that backstory. Seeing media thru other eyes, or studying more on how it got me, opens me to new perspectives on the things I love. Sometimes, like in relationships with actual people, I love it less, it's flaws made more manifest. Sometimes, I love it more; every time I re-read or watch a good adaption of "A Scandal in Bohemia," I just fall again for how Conan Doyle just...lets Holmes fail, and learn something along the way. It's amazing work on every level.
So: it's not just possible, but worthy, to revisit media and look at it in new ways, and with the understanding of accumulated knowledge. You can even say "hey, I didn't realize 20 years ago this was problematic, but now I agree with the criticisms people have raised, over those years."
People are allowed to change their minds. Even Ebert did, and wrote on why he changed his opinions, more than once.