While not exactly what you were suggesting, DDR4 ECC DIMMs and SO-DIMMS are already "pin compatible" with non-ECC modules. So it is straightforward to design a

While not exactly what you were suggesting, DDR4 ECC DIMMs and SO-DIMMS are already "pin compatible" with non-ECC modules. So it is straightforward to design a CPU and motherboard that support both, with a control bit in the IMC allowing the BIOS to enable/disable ECC checking (and the extra ECC byte lanes) iff ECC modules are installed (and that's what AMD did with Ryzen). It is fair to say that Intel blocks ECC on its desktop i7 parts for non-technical/business reasons (i.e. so they can sell a higher-profit E3 Xeon if you want DRAM data integrity beyond what non-ECC memory can provide).