You need to change engines, Unity will not going to be in control for long and there are better open sourced engines available to easy redo your project in a few days with C# Programming. 

This by itself is not very convincing; it is like saying "Just install Gentoo, you can still play all your Windows games on it."  This is more likely to take months of dedicated work, with bugs and unforeseen obstacles continuing to crop up through the life of the project.  Can you speak to the ease of migration to any of these engines from firsthand experience?  If so, I would like to hear the details of what was easy and what the pain points were.  Just skimming the documentation for Stride and NeoAxis, I see several incompatibilities that would require significant time and effort to refactor.

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.

I have a game made with Unity called "Randy & Manilla", and to this day i still use the Unity 2019.2.9. But after the news of the merge with Ironsource, I was forced to change the engine for 2023, but I don't know which one to choose. ¿Unreal, Godot or Flax? Speaking of Flax, I've heard that it has a very similar interface and scripting language to Unity. But nevertheless, since I have it in development since 2019, what do you think is the best option?
Change the engine, or keep the existing one to finish it?

I must the panic attacks from this are a bit overblown considering a couple of facts:

1) InstallCore has been deprecated for a while. If you try to google for it or it's SDK you will find absolutely nothing. 

2)InstallCore itself wasn't a virus at all as many people think it was. It was actually an open source installation and content distribution SDK which is where it was classified as a virus because many malware creators used it to create infected installers for programs. Not to mention it was also free and widely used(100 million users according to wikipedia).

3) Ironsource shifted their focus towards mobile app development and monitization and their main area of focus nowdays are mobile apps , ads and games. They even own a mobile game publishing company called Supersonic studio. Many mobile developers use Ironsource for their mobile apps or games even the ones made in Unity. 

Unity itself will not become a virus or become overloaded with ads. The reason why Unity merged with Ironsource to begin with was because Unity started to focus more on profit and mobile ads were earning unity a lot of profit and that's where Ironsource is hugely successful.

This merge will only massively change who the provider of mobile ads is going to be thou in reality nothing will change with this merge because you could already use the Ironsource SDK with Unity for a long time(since version 2017.2).

You can still use any older Unity version if you so desire. The only reason for change is because Unity as a company themselves shifted focus from the game engine to profits. The rest is honestly just fearmongering. While IronSource is shady rest assured that this will only affect the mobile game development and mobile ads.

Quite honestly the reactions I am seeing to this news around the internet, including from the media, concern me more than the news itself.  There is a narrative going around now that games made with Unity are dangerous and must be avoided because they are all going to come bundled with malware from now on.  All this does is hurt the indie developers that build with Unity by promoting disinformation about their products.  We shouldn't be your targets.