Some Ryzen motherboards are marketed explicitly as having ECC DRAM support (e.g., ASRock Rack X470D4U, ASUS Pro WS X570-ACE).
Other Ryzen motherboards have ECC DRAM support, but they don't advertise it as a feature for one reason or another (most of ASRock's motherboard lineup, many mid-range ASUS motherboards, possibly others).
Still others completely lack ECC support (many low-end boards, as well as high-end gaming-focused boards).
> what does enabled but not validated mean?
Basically, it means that it's optional. As described above, ECC can work or not work depending on which specific motherboard you get, and it's up to the system integrator to choose a motherboard compatible with ECC if that's a desired feature.
That's as opposed to "validated" ECC support in, say, AMD's Threadripper platform. In that case, if a company builds and markets a motherboard as compatible with Threadripper, and it lacks ECC support, they can expect to receive a nasty letter from AMD's legal team.
Are you sure about that? I'm only asking because the System76 Threadripper Thelio[0] doesn't support ECC (according to the response I received from their support people). Their response was actually that "Threadripper and our motherboard do not offer ECC" (TR obviously does support it though), but is it the case that they're actually contractually obligated to support ECC?
The Thelio Major uses a Gigabyte X399 Designare EX motherboard, which has ECC DRAM support. System76 may not offer or support ECC DRAM as an option, but you can add it yourself if you're so inclined.