AFAIK the Z80 hasn't been publicly reverse-engineered to transistor level completely, it's mostly some important pieces (those that Ken Sherriff did, mentioned in the article) but apparently that was enough...
Notable that one of its direct predecessors, the Intel 8080, was reverse-engineered to transistor level:
Notable that one of its direct predecessors, the Intel 8080, was reverse-engineered to transistor level:
https://zeptobars.com/en/read/KR580VM80A-intel-i8080-verilog...