I got the email and there is not steam key option in my downloads (You claimed this game Jan 10, 2023 for the promotion The Arrival of Nightwind.)
Hi! Recevied the email means you are one of the "Download keys" owners, and you can access the game forever and its external key (Steam key). Even if you claimed the game in a 100% off sale in The Arrival of Nightwind. As stated in this article: https://itch.io/docs/creators/download-keys
I suggest you refer to this page to generate a new link for the download page: https://itch.io/docs/buying/already-bought
The "Claim Steam key" button should be right below the download section.
If you keep having problems with itch.io support not getting back to you, I suppose you could get Steam keys directly from Steam and send them by email to all owners of the itch.io game? If that were the case, it'd be also be a good idea to send an email to customeres to ask if they are interested in one and to have them reply with their unique download link to confirm their ownership of the game.