The wording from AMD is pretty strange, frankly: the official pages for the APUs (7640U, 7840U) now say “ECC Support: No”, but they used to say “ECC Support: Yes (FP7r2 only; Requires platform support)” back in May and I seem to remember even in summer, though I don’t have a copy (here FP7v2 is the DDR5 SKU, the other two are LPDDR5 ones).
I don’t know what happened. Either AMD screwed up the circuitry and only has enough units with working ECC for the PRO bin (I doubt those are actually different chips), in which case we’re just in an unlucky generation; or they are following Intel’s lead and using ECC for market segmentation now, in which case we’re all screwed and I in particular am greatly disappointed. I don’t know.