We have a section in our community guidelines about this: https://itch.io/docs/creators/quality-guidelines#avoid-only-uploading-keys-or-links-to-other-stores
Generally we discourage developers from uploading just a link to their Steam page. Empty pages like that would be de-indexed by default so it’s not going to help you get more attention to your project (unless you already have followers that would get notified).
We do understanding that Steam provides online services that we don’t, so in some cases we make exceptions and let people only sell Steam keys. By default though, for a page to be purchasable a file must be uploaded.