Someone could have official terms and conditions with proof about the date. Such as for example receiving them by email or having them archived on a reliable website.
When it comes to reliable site for archiving of such data then I guess Archive.org's way back machine could be the most reliable site to search for old archived pages of the sites. You may try to search there and there's chance you might find something there. However, I think that you're searching about terms and conditions of 2021 and that's why there might be some inconsistencies or errors whatsoever.
Since terms and conditions don't come in pdfs, at least for the most online platforms available, it's hard to find such saved up somewhere, I do not know much about people achieving such on sites like achieve.com, but the big question should be why would anyone want to do that? It possibly won't occur to any one back then that some day, the some one somewhere may be in need of that very version of the terms and conditions of that casino, so, I highly doubt any one would have saved up the terms on the time machine.
Like I said before, the such information, the best bet to get it is by contacting the customer care, but then, the chances they will give such information to the gambler is very low, since they will think the gambler may or might use the information contained there against the casino.