This chart shows all releases in the Unreal Archive by estimated release date. Estimated because it's based on file timestamps within the zip/archives of the content since most of it doesn't have an easily findable original release date obviously. So repacks and things will appear later, and people who packaged things with incorrect times will appear earlier (you can see UT2004 releases back in 1998 for example

Anyway, thought it was interesting to see the cadence of each game's rise and fall. Only UT's content creation has continued reasonably well throughout the years.
