Which OSes are actually imposing DMA restrictions on internal cards? That feels like something that would impose noticeable overhead, but I guess I can imagine a special mode that enforces this for competitive gaming...
It's not a global on-off switch. With a proper IOMMU, the hypervisor/operating system can lock out specific devices from DMA access, or confine them to specific address ranges.
Allegedly some of the anticheats are configuring the IOMMU through Windows APIs (vanguard, faceit, and a smattering of chinese anticheats). It’s hard to find good public information though. They do some mix of blocking access and deliberately leaving some pages as bait (and monitoring iommu d-bits/faults)