> This is present on some DDR4 and DDR5 modules, but information on this is kept internal by the DRAM makers and not usually public.

On-die ECC is going to be a standard feature for DDR5. I'm not aware of any indication that anyone has implemented on-die ECC for DDR4 DRAM, and Hynix at least has made clear statements that on-die ECC is new for their DDR5 and was not present in their DDR4.