As long as you have the Retina 4K version of the 2019 iMacs, you can upgrade the CPU to ‘theoretically’ anything in the Intel 8th and 9th gen line. If any of you want to actually put your 'wise words' to work, you will see that someone on Youtube has already upgraded their 21.5” to a 9900T. https://www.youtube.com/watch?time_conti... 8 Cores, but a lower TDP. Yes, it works. Yes, it can be done. How well does it work? There was no update video over the last 4 months, so I imagine nothing broke. Does that mean the rest of 9 series will work? Theoretically now looks a lot more like very possibly. Will there be issues with TDP or other issues? I don't know because I 'HAVEN'T TRIED IT' and I wish others would preface their comments the same and quit spamming every question with answers they pulled from 3 seconds of googling Apple’s specifications pages.
From what I’ve experienced through Apple, they don’t actually do a lot of ‘repairs’. They can definitely diagnose and replace components, but in terms of circuit level components, they just up and replace everything with a new part. They call these ‘Depot Repairs’. If you actually pay apple to fix liquid damage on your board, they will not even mess with any ultrasonic. They may not even have such machines in a Genius Bar (I can not verify that statement). So what you are asking doesn’t apply to Apple, so you have to use the thinking of, the indicator was already tripped when something was spilled. Your use of ultrasonic will mark that it was repaired properly.
If you browse the Mid-2012 Geekbench 4 results, you'll find that all of the highest scores have 2133MHz RAM inside of them. While this may likely just be coincidence that power users put in the best possible regardless of benefit, it does guarantee compatibility. Again, it stands that these systems can only address 1600MHz, and any faster will be slowed down (unless I am missing something). Might be a good experiment.