DMA is "easy" to patch. No reason to allow a device to have arbitrary memory access. Just require use of IOMMU.

FaceIT essentially has countered most modern cheats including those using DMA. https://www.faceit.com/en/news/faceit-rollout-of-tpm-secure-...

Nowadays if memory access is needed, you are looking at having to find a way to load a custom BIOS or UEFI module in a way that doesn't mess with secure boot. Even then, certain anti-cheats use frequently firing interrupts to find any unknown code executing on any system threads.