Twitter already had a liquidity event, the IPO. A going-private transaction is unlikely to be contemplated in any existing options or RSU agreement. Unvested options are technically worthless, being an option to purchase securities that will no longer exist (TWTR). In practice it would be very silly to do this to employees unless your explicit goal was to massively reduce headcount.
Lawyers contemplate lots of things in multi-page agreements that most people don't read :-) The two times I've been acquired (by a public company but I think cash deals), unvested stock has been converted to the acquiring company's stock at the equivalent value and on the same vesting schedule. Of course, this will be a private company so the situation is different.
