This is kind of an overblown take. I don’t want to minimize the fact that Ironsource is awful, and given current trends in management it’s probably best to drop Unity as soon as convenient, but the engine will not magically transform into a virus overnight. Far as I’m aware the company also no longer produces malware, pivoting to the similarly-awful business of mobile game monetization.
Does that make them trustworthy? No, absolutely not. But I don’t think the level of “Unity’s turning into a virus!” fearmongering going around right now is healthy, you can criticize this merger and question the engine’s long-term viability without crying wolf.
Most long-running projects generally don’t change engine versions often, if someone has a game in development they can complete it (and probably should, given the monumental cost of switching) in the same version they started in. After that? Probably a good idea to look at alternatives. But there’s no need to jump ship immediately and risk your project in response to this news either.