IANAL, but my understanding is that these contracts are usually stated in such a way that what Mr. Musk is doing is not a breach of contract. If he was literally withholding wages then of course that would be illegal. But if the contract simply says something like "All persons who are employed by Twitter as of Nov. 1, 2022 are entitled to a stock grant" then firing people on Oct 31 is a completely legal way to avoid giving the stock grant. It may be a dick move but that is a separate question from whether it is legal.