The biggest problem there was programs hiding away state and then not masking it out when using the addresses. This meant they couldn't just use the lower part of memory on new chips, they were completely incompatible. The lesson has been learned from that, and CPUs don't ignore reserved bits.

But also this is a hardware feature so the code will continue to be compatible anyway.

Even if this acts like a permanent truncation of the address space by 4 bits, so what? If you think you can keep Moore's law going strongly enough to hit that cap, you're probably going to hit the 64 bit wall soon enough after; might as well focus on that shift to 128 bit addresses