Sorry for the slightly delayed response.
First of all, thank you for your answers.
"Other person could make a similar game, except the task manager is always locked, even inside the game.", okay, but in itself, my game would be legitimate, it's the other person's game that will be the virus. I don't really see the connection that has with my own game.
You also say that a virus could infect my game and use this game function to break the computer, but I don't really see how that would work. Now, I'm not exactly a virus expert, but how could a virus infect my game and cause the task manager blocking to work at any point during gameplay? If what you mean is that someone could, for example, create some kind of mod for my game, which the player would then put in the game's folders to modify the code and make the task manager permanently blocked, then I understand that it could be a problem. But then, the mod would quickly be flagged as a virus and banned by itch.io or any other platform that shares mods. And if the mod is on a site that doesn't moderate what's posted, then the site itself is breaking the law. Furthermore, if the mod becomes even slightly well-known, I'm likely to hear about it, and in that case, I would take it upon myself to remove the mod.
As I said, I'm not a virus expert, so I could be wrong. I should definitely look into this further.
Otherwise, if the functionality is truly problematic (and from what you're telling me, it seems to be), I'm trying to implement an alternative system (especially since closing the task manager in Windows is quite complicated).
Instead of the game blocking the task manager, it simply creates a separate program at launch whose sole purpose is to constantly monitor whether the game is open. If the game is closed, the second program will relaunch it. Similarly, if this second program is closed, the game will relaunch it. Technically, this doesn't require administrator privileges, doesn't block or modify anything on the PC, and furthermore, the player can always close the game by selecting both programs simultaneously in the task manager.
So, to answer the question of "why do I want to do this when most players (probably 99%) won't bother trying to use the task manager", it's for two reasons.
The first reason is so that the game has precisely this "virus" effect (even if it won't really be one) by giving the impression that the game cannot be closed and that it has control over the player's computer, a bit like what KinitoPet does.
The second reason is that I want to put in a sort of "semi-jumpscare" that's a bit hidden, basically a kind of easter egg where the game character slowly leaves the game window and slowly approaches the player's screen and does something like point at the task manager window with his finger and shake his head no, as if to tell the player that it's pointless trying to close it and that the game has total control over his computer (he doesn't jump on him either, that's why I say "semi-jumpscare"), in itself it's not something necessarily useful and surely very few players will notice it, but I think it can still be something really scary in the moment.