Intel also used the 2-bit-per-transistor ROM in the iAPX 432, their unsuccessful "micro-maninframe" chip.

Nowadays, flash uses multiple voltage levels to store four bits per cell (QLC, Quad Level Cell), which is a similar concept.

I wrote a whole blog post about the 2-bit-per-transistor technique, back in 2018: https://www.righto.com/2018/09/two-bits-per-transistor-high-...