Good question, which I have had myself seeing as I'm trying to build a new PC with a 9800X3D. Just waiting for the ludicrous 9800X3D prices to drop. They're about 640 USD here right now.
What I've found on
Reddit:
Ryzen 8000 and 9000 CPUs support CUDIMMs in "bypass mode". This means they will function without the CKD, so there is no benefit to paying more for it. Additionally, they will boot by default at DDR5-3200 (the lowest possible JEDEC speed) and will require manual tuning to run faster.
Don't buy CUDIMMs for use with AMD CPUs. As stated by an AMD employee on this very sub, the "support" is really just to a level of tolerance that avoids unusability.
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Better to keep aiming for regular DDR5 DIMMs.. Seems like a terrible time to upgrade or build a new PC.