Absolutely not. Creating a culture where employees are expected to be silent about their (mis)treatment by wealthy owners is only favorable, to, well, wealthy owners. Business is business, so why is it unprofessional to point out they're bad at business?
If a company wants to buy another company and sunset it, that's a normal business practice. I get that it's disappointing, but in no way is this "mistreatment." At least to me, this is a perfectly normal business situation that doesn't merit this level of complaining. It reads as an ex-employee being petty.