Itch.io does show me a banner at the top of games that I've purchased or supported. If you're not seeing that then it may be that it's a duplicate of the game or the developer deleted the game page and re-uploaded it - (they're not meant to do either of these two things).
Else it could be that you got a game at 100% off that wasn't claimable.
Under More information on the game pages - there are 2 dates - a date on which the game was published and a number of days since it was last updated. Probably an update date would be more useful than the number of days - but you'd be able to tell from there if that's the version you have.