note: I am not (intentionally) endorsing necroing forum posts.

I after a long few minutes of toiling away at my search bar, i have found the first ever forum post (that has been locked at least) It is called: Water Effect

https://powdertoy.co.uk/Discussions/Categories/Index.html?PageNum=2387#RecentTopics

Likely to prevent necros, it stops you from just clicking the number at the end of the bottom, but i figured out how, you can use the searchbar, and go to PageNum=N and reach a number, i used google trends to try to figure out what the date would be too

it had a lock "preventative lock" as if it may have been the first thread, but the first thread is likely long necroed