I once scraped a subset of all Slashdot posts for the month of October. There were some interesting trends. The bulk of posting is by old UIDs in the 100k to 300k range and those are dropping off rapidly, suggesting that it will die around 2022. Some of that is attributable to the rise of mobile and more frequent anon posts but that doesn't balance out the decline from logged in posts.

There is also evidence of UID inflation by only issuing odd or even numbers during certain periods.