I assume most anti-cheat systems will sooner or later require Secure Boot and TPM, which makes a lot of sense. Additionally, it's more secure to have them enabled, and similar solutions have been used on iOS/Android/macOS for a long time now.

Many Linux distributions have signed bootloader and kernel, to support secure boot, but otherwise I think you could either add your own signing keys for the Secure Boot, or chain either Linux or Haiku from a signed grub bootloader.