The only thing that might give me pause is that there's no conciliatory language coming from Mongodb to indicate that it was a negotiated exit, not even a bland "by mutual agreement".

In fact, they seem to have gone out of their way to minimise any mention of him which, to me, suggests that it might have been worded that way based on legal advice rather than for PR reasons.

In the US, there is only one labor law that is different for execs and high paid employees and that is overtime rules. Fiduciary duty does govern how a director or officer behaves while in the role, loyalty, no self-dealing, no stealing corporate opportunities etc. It doesn't oblige them to stay or dictate how they must leave.

In US and English law, "specific performance", so a court ordering you to do what you promised, isn't available for personal service contracts. In the US that's reinforced by the 13th Amendment's ban on involuntary servitude. So even if a CEO signed a contract promising 6 months notice and a smooth handover, the company can't make them do it, they can only enforce any financial penalties that are contracted.

I have never heard of this. Do you mind sharing some examples?

I'd never heard of this. It turns out it is very rare for publicly traded companies [1]. In these cases, I would guess it comes down to contract terms. But again, it's not as though the law would compel an individual to continue working at a company. It would just come down to what the contracts say, and, potentially, how the courts interpret those contracts in the event of a lawsuit.

[1] Examples I found: Worksport Ltd., Exicure, Inc., Rainmaker Worldwide Inc., and Acorn Energy, Inc.

it simply works, that's how you optimise tax on million+ sums, do you think ceo-s pay taxes on par with regular workers?

A contractor CEO? I know it is extremely common for directors to be contractors for tax reasons.