It's not uncommon for companies to "clear house" right before close of acquisitions, especially on exec level. However I don't believe it's for the reasons you mentioned (saving money). Instead it's likely to better align with new ownership (whether perceived or actual alignment) and ensure the company is well positioned for the change.

An established/relatively healthy org like Twitter will likely not be penny pinching at the risk of more fallout/attrition. In fact, I'm sure the convo went something like "if you leave now, you will keep xyz/golden parachute".

This has at least been my experience based on limited experience of being part of a few acquisitions and working closely with execs.