What's the status of ECC support on AM5

Level1Techs forum is likely the best place to get this info, because Wendell is active in testing these things.

As far as I know, AsRock _originally_ spec'ed their motherboards to support ECC memory, but then backtracked after it was discovered initial AGESA from AMD did not support it.

AMD eventually updated it to support ECC, but AsRock took a long time to revert their motherboard spec pages. Eventually, though, they did revert. So they again state that ECC is supported.

And as far as I understand it, the support is pretty good.

But if you absolutely want to make sure this is true to the greatest extent possible, you can always buy an AsRockRack board, along with an EPYC 4000-series, rather than Ryzen.

Me personally, would buy regular AsRock board, as they actually have better support (BIOS, etc.). I'd have chosen EPYC 4000-series over Ryzen, but they don't have a 7800X3D equivalent, so it's a no go there. Maybe they'll have an EPYC 4005 series that is an equivalent of 9800X3D. All future stuff.