Not all of them. Have a look at a Raspberry Pi or Apple Silicon devices. In fact most ARM SoCs I am fairly sure don't use PCIe for their iGPUs. This makes sense when you think about the unified memory architecture some of these devices use. Just in case you aren't aware Proxmox does indeed run on a raspberry pi, and I am sure they will gain support for more ARM devices in the future. Though I believe an x86 device with unified memory could also have problems here.