The vendor claims that the article with product no. CT2K16G4S266M is 100% compatible >> https://www.crucial.com/memory...

Just worked in my new iMac 5K 27" 2020 (3,3GHZ 6-core AMD Radeon Pro 5300 4GB).
Was 8GB, I added these in the two empty slots and it now shows 40GB.

I bought this RAM Kit for the newest iMac 5k 2020 (i7 10th gen) and the RAM Kit does NOT work! I tried different slots and even with just one RAM slot it was not possible to get the iMac to boot.

I would highly recommend not to buy this kit until someone has rejected my statement.

It works perfectly with my 27-Inch 5K Retina Core i5 3.7 GHz which I bought new from Apple earlier in 2020, although I think technically it's a 2019 model (iMac19,1). Hope that helps!