Relatively common nowadays. It used to be delineated as a feature in Intel chips as part of their vPro line, but I think it’s baked in. Generally an IOMMU is needed for performant PCI passthrough to VMs, and Windows uses it for DeviceGuard which tries to prevent DMA attacks.
Mainstream consumer x86 processors have had IOMMU capability for over a decade, but for the first few years it was commonly disabled on certain parts for product segmentation (eg. i5-3570K had overclocking but no IOMMU, i5-3570 had IOMMU but limited overclocking). That practice died off approximately when Thunderbolt started to catch on, because not having an IOMMU when using Thunderbolt would have been very bad.
