N64 is well understood, and there are low level emulators now available that emulate the RSP and RDP on modern GPUs implemented with compute shaders. https://www.libretro.com/index.php/category/parallel-n64/
That's relatively recent, though; the vast majority of the emulation knowledge was from reverse engineering, patents, and typical devkit leak stuff as you say.
TBH, most peoples still prefer to RE on dedicated N64 hardware (hand crafted physical CPU interrupt/inspection). The problem is Verilog source alone is not that useful as you need to “plug” it to CPU with the N64 interface.
"This far back" is not that far back. N64 came out in 1996, at which point custom ASIC design and EDA tooling was already commonplace.
Verilog appeared in 1984, and EDA tooling was in use way before that.
What's more commonplace today is the number of fabless IP companies in the semi space that focus exclusively on selling only IP, rather than selling fully made chips, like SGI, NEC, TOSHIBA, etc. did back then.