There was a certain level of custom HALs being written, but those depended heavily on the "shared development" model that Microsoft did with vendors, and a lot

There was a certain level of custom HALs being written, but those depended heavily on the "shared development" model that Microsoft did with vendors, and a lot of hardware was shipping bog-standard PCs anyway. The rare custom HALs were mainly for things like x86 NUMA systems, or from ports to alternate architectures.