All I have seen on Kupp is that he got a three year extension so he's signed thru 2023 - no indication that I've seen if it's a straight extension or if they absorbed the last year of his rookie deal. They might have scrapped the last year of his rookie deal (like they did with Ramsey), but if they did they replaced it with a 4 year deal.
With Ramsey you can see the mechanics of that (scrapping the last year of a rookie deal). He got a 5 year extension but it was written as a 6 year deal, rolling in the last year of his rookie deal.
Here's Ramsey's deal showing how his 5-year extension was implemented. (Note that his salary in 2020 was $13.7M before the extension.)
AlbaNY_Ram