I'm pretty sure SRAMs 1:1 ESP system first appeared in 1996 with the ESP 900 mechs and gripshift. It was certainly around in 1997 when they upgraded it to ESP 9.0.
The same 1:1 cable pull ratio has been used by SRAM ever since, so it is possible to use old mechs with modern shifters and vice versa.
You do still need to make sure that the shifters and mech are 1:1 ratio though, as SRAM still made and still make shifters with the Shimano compatible 2:1 ratio too.