Nope; the change happened in the 386 era; by the 486 era most BIOSes would be (partially/mostly) written in a pretty uncarefully written "HLL". Even before I would argue against the "carefully optimized" ASM.
Contrast that with just shoving compiler output into the ROM image.
Do note that, even with compilers of the era, using HLL with care can result in having an _easier_ time managing codesize than some of the garbage written in assembly by random developers... specially as feature creep starts to set in, which by the 2000s was in full force.
Have you ever seen a USB stack (even for HID boot protocol) in assembly on a commercial BIOS?
Yes. On AMIBIOS 627.10 and Award v6.00 (both used into the early 21st century) the USB stack is 100% Asm, as is the rest of the BIOS.
I left that scene in the late 2000s but my understanding is that C didn't start showing up in BIOS until the UEFI era (Tianocore etc.).
There have been some leaks over the years of both "old-school" and "new-school" BIOSes, so these facts can be verified.
> C didn't start showing up in BIOS until the UEFI era (Tianocore etc.).
"Showing up" is an understatement, since Tianocore is almost all C. And we didn't switch from 100% asm to 99% c overnight.
It wouldn't fit in a 64K ROM otherwise, and even then it was compressed.