A lot of these labor laws get less protective for higher-ranked people in a company, whether it be due to thresholds based on compensation or explicit carve-outs for executives/board members. It varies a lot from place to place.

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.