Theoretically, installing a catch can should not void the warranty unless it causes the problem. However, from the reports we read here on this forum and others, if a dealer or warranty representative is looking for a reason not to cover a problem, they can possibly use any aftermarket item added to the drivetrain as their reason. You may be able to eventually override this or go to another dealership, but it still creates a huge hassle.
To me, the part of your question that is the crux of the matter is the phrase "if I have to use it". I've owned several vehicles with Hemi engines over the years. My current daily driver is a 2006 Charger Daytona. It NEEDS a catch can. And I have to empty out the milkshake often. But since the days of these early engines, there has been a lot of improvement to eliminate blow-by and loss of oil through the PCV system. My later Hemi engines have never needed this and there is basically no oil loss this way. So on your 2020 Ram, why do you "need" a catch can?
If I actually didn't need a catch can but simply wanted one, I'd likely just resist the urge as long as my engine is under warranty. While the catch can would technically void any unrelated warranty work, why take the chance? The PCV systems on the later model Hemi engines is just fine as is.