Highly unethical would be throwing the CEO of the division keeping the lights on under a bus with zero regard for the consequences.
The non-profit board acted entirely against the interest of OpenAI at large. Disclosing an intention to terminate the highest profile member of their company to the company paying for their compute, Microsoft, is not only the ethical choice, it's the responsible one.
Members of the non-profit board acted recklessly and irresponsibly. They'll be paying for that choice for decades following, as they should. They're lucky if they don't get hit with a lawsuit for defamation on their way out.
Given how poorly Mozilla's non-profit board has steered Mozilla over the last decade and now this childish tantrum by a man raised on the fanfiction of Yudkowsky together with board larpers, I wouldn't be surprised if this snafu sees the end of this type of governance structure in tech. These people of the board have absolutely no business being in business.