500 is an arbitrary cutoff. As far as I know the number has never changed in the history of the site. But in one sense the avatar requirement has been lowered over time, simply because the opportunities for participation in discussions are much more numerous than in the past.
There were about 1520 new forum threads in the month that I joined MacRumors in 2002. In comparison, last month there were about 17300 new forum threads, i.e., over 10 times as many discussions going on at any given time. It's much easier for almost anyone to find conversations of interest. The 500-post requirement used to represent years of participation for most forum members, and now it sometimes represents only months of participation. Avatars identify the most active users, who once numbered in the dozens, and then in the hundreds. Now thousands of users have avatars and people earn them faster than before.
Meanwhile, all users can give themselves a
profile picture that will appear on their user page.