This is my guess. He probably knows a sale is happening and doesn't want to provide value to the new owner. Probably because he has a competitive portfolio company or something.
Also, as valuable as his presence is, I can't imagine a buyer of the size needed to buy Twitter would be like "we'll give you n Billion, but only if pmarca stays on point"
Not sure why he would delete the (public) history, though.
He was pretty notorious for blocking anyone with an opposing viewpoint on Twitter, so this seems in line with that.