An employee or insider can cancel their participation in the Plan. It's kind of a legal grey area and certainly doesn't look good if something like a buyout happens shortly after. So even if it's not illegal, it carries some reputational risk.
Just to be clear, there's no reason to assume this means a sale is imminent. It's mostly the Board aligning the interests of the CEO and shareholders by making sure the CEO doesn't resist a sale. So to be more precise, I'd say it's more a prerequisite to any sale than a signal of a sale.
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