That's what I'm talking about and I wouldn't call it "fully supported". I want to know about ECC statistics. It's important because if I can see that ECC recovers abnormally high number of errors, it's likely that I need to replace RAM right now.
So on Linux, ECC is fully supported, even with Ryzen.
https://www.servethehome.com/asrock-rack-x470d4u-review-amd-...
With Ryzen 3xxx, I have used ASUS Pro WS X570-ACE ($315), which is sold as a workstation board, so you definitely should expect ECC to work without problems, and also the Mini-ITX MB ASRock X570 Phantom Gaming-ITX/TB3 ($230), which also worked OK.
I expect that the other ASRock MB's (most of them or maybe all of them specify the support of ECC) also work OK with ECC modules.
The ASrock Rack server board should also not have any problems with ECC. IIRC that server board supports only up to DDR4-2933, but until now faster unbuffered ECC memory modules were not sold anywhere, so that is not a disadvantage.
Edit: I have looked again at the ASRock site and they have updated the memory support specification. If you use only 2 UDIMM modules (i.e. up to 64 GB total), then you can use up to DDR4-3200 (which I have never seen offered anywhere until now; 2666 is easy to find, 2933 is also supported by Intel since March, so it should become available soon).