It varies. I'll preface this by saying I am not privy to the specifics of this deal, but in general this depends on the terms of the options plan. Some employees may have their options accelerated upon triggering events like a change of control. Others may have options accelerated on a "double trigger" such as a chance of control followed by termination.
> And what’s my incentive to work for an established private company with no stock growth prospect? I guess Twitter will have to pay 500k senior eng. salaries in cash instead of stock?
You don't need publicly traded securities to have deferred compensation. Shares continue to exist even if the company is private and liquidity events can be structured to allow employees to cash out in a "controlled and deferred" fashion. Plus you could structure deferred compensation entirely without those liquidity events or shares for that matter. Just promise to pay people $X in Y years
From a "Twitter is a going concern" perspective, the company and its technology function today. In a non-functioning state, the assets have materially less value. As in effectively zero value. Therefore, it is in the interests of the new controlling owner to see to it that enough of the current employees remain at least long enough for a decision to be made about which of these employees are wanted long-term.
As a result, I anticipate that efforts will be undertaken to see that employees are made close-enough to whole that there is not a larger mass-exodus than the change in the political party of the senior leadership is likely to generate on its own, more or less regardless of whether the employees have any shareholder rights in their RSU contracts.
Typically insta-vest and converted to cash on date of sale.