Maybe I was lucky, but I had the same issue where I no longer had access to my old e-mail account and they sorted it out for me within a few hours (after a few e-mails back and forth).

It probably helped that I was able to provide them with a screenshot of the game in my Steam library as well as a list of all the games linked to my Rockstar account with dates of purchase. To find the dates of purchase on Steam I went to “account details” and “view purchase history”.