“We talked about it [making Classic] for a number of years and we couldn’t figure out what the right way to do it would be,” Brack explained. "The problem was, the codebase that shipped with WoW and encompassed the WoW Classic timeline was a codebase that we had really moved on from in subsequent years. Software is a very iterative process, and its development was more than 10 years in the past at this point. So thinking about the hardware that didn’t exist anymore that we would need to run on it, running two MMOs simultaneously with two different toolsets, two different toolchains, it was not something we could really get our head around.”
HOWEVER, I did find out that there was an exploit to get around it (one of those rare exploits where you would WANT a bit of lag). Basically if you spammed “X” to sit/stand/sit/stand, then if a mob hit you while you were sitting and right as you were standing up, then it would register as a crit, but on the server side you would be standing, thus allowing the ability to proc.