Blocking the task manager isn't illegal in itself.
What is illegal is doing it without the user's consent. Furthermore, this type of behavior is typically associated with malicious programs, so your application will trigger antivirus software, be blocked, and possibly quarantined by Itch.
You might think it's a good idea and a great way to break the fourth wall, but trying to limit the basic functionality of someone else's computer is rude. There's no justification for doing so, and in the long run, it will only cause you problems.
Why should anyone trust you to block basic security components of their computer?
All you're doing is raising suspicions and raising concerns that will lead to your program being flagged as malware.