The most common sense thing that you probably already know is going to any still existing abdl sites with an image gallery that existed before 2008 and sorting the images by ascending order. Sites that go that far would be diaperedanime, exhentai, imagefap, dailydiapers image gallery, ararchive old image gallery and Abkingdom image gallery but they charge money to access older images. Abkingdom also has a links page which might help find some still existing abdl sites with image galleries.
Not sure how much pre-2008 art tehthul had but when the admin still had their reddit account active. They mentioned still having access to the drive with all of the photos and art intact. Not sure if we'll ever see the admin show up again and try to convince them to at least post all of the lost art. Trying to scrape what the Internet Archive was able to archive from the site didn't yield much. About 1GB of content with most of it being thumbnails. IA was complete shit at archiving Deviantart content so that wont help you find old art. FoxTimesTales had the image gallery locked behind a login and any images posted in the forum needed a login to view them so IA didn't save any of that art. Only the art that was only posted on the front page of the site. I have not bothered to check if any 4chan /d/ archives exist anywhere. Furthest I could get was 2012 with no art saved from the threads and I know that diaper threads existed by at least 2008 when I visited the site.
We also already have an old art thread here
>>15667 but had it be pre-2014 since I didn't want it to be a dead thread with how little art I had that was pre-2012. The quality of this board was starting to get worse by then so I wasn't sure if I was willing to put anymore effort posting decent old art. First three images being from 2007, 4th 2008 and I remember seeing 5th pic around 2008 but I think it's slightly older than that.