The official reason given is "he was fired because of the egregious nature of the findings in a misconduct probe".

Supposedly there are rumors of sexual misconduct but I've not seen any official statements or evidence to support those rumors.

Beau denies those rumors and says it's a smear campaign...which is what someone who was guilty and trying to hide it would probably say but also what someone who was the victim of a smear campaign would probably say. Looks like he has a lawsuit against Marvel in the works but who knows what will be made public.

So he was fired and had his season 2 credit stripped for "Egregious Behavior", to which he countered that he was fired for being out and proud and posting shirtless photos of himself...

Then this happened:

https://comicbookmovie.com/x_men/x_men-97/x-men-97-shocking-new-details-emerge-about-beau-demayos-alleged-sexual-misconduct-working-on-marvel-series-a212686

"It's claimed that DeMayo sent nude photos of himself in sexually suggestive "hero" poses to several young male staffers working on X-Men '97. His excuse was that they could be used as "inspiration" for the animated series (DeMayo also runs a non-explicit OnlyFans account which is previously said to have upset Disney). 

The Head Writer was told to stop sending these photos but allegedly continued.

Sneider also claims that DeMayo groped an assistant on multiple occasions and was considered emotionally and physically abusive towards staffers."

X-Men '97 legit rules, you should watch it.

It's a fun show and he's no longer involved. Seems like a good reason to watch it to me.