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My guess would be closing stages of an acquisition, code audits etc. etc.

What acquisition makes a company completely stop working on product, cancel events, and stop social media?

Most of them.

Perhaps. My experience (a data point of one) wasn't like that. Why would you risk team productivity until the deal is signed?

Depends on what you're doing with the team. Going dark is not, for instance, a strong indicator that they're going to keep the product alive.