Their first site (haveibeentrained.com) was offering a way to search through all the training datasets, not realizing, they were full of illegal porn - so it was quickly shut down.
Now their main gimmick is offering a browser extension, that lets you see what data on any given site you visit, was used for AI training, what has already been marked as "opted out" and a way to add your stuff, to that list.
I don't like that idea either, adding URLs to a list, should not require questionable browser extensions and in general, opting out all the places that might have your images, doesn't seem worth the time, if the companies, don't even have to respect this request.
If you just want the txt file, without additional nonsense, feel free to take the default one, that I use here: https://thecanine.ueuo.com/ai.txt and use or edit it, to match your needs.