> Was there some relatively recent fundamental breakthrough or other change that prevented a Fil-C-like approach from being viable before?

The provenance model for C is very recent (and still a TS, not part of the standard). Prior to that, there was a vague notion that the C abstract machine has quasi-segmented memory (you aren't really allowed to do arithmetic on a pointer to an "object" to reach a different "object") but this was not clearly stated in usable terms.

Also in practical terms, you have a lot more address space to "waste" in 64 bit. It would have been frivolous in 32 and downright offending in 16 bit code.

The memory model always had segmented memory in mind and safe C approaches are not new. The provenance model makes this more precise, but the need for this was to deal with corner cases such as pointer-to-integer roundtrips or access to the representation bytes of a pointer. Of course, neither GCC nor clang get this right, to the extend that those compiler are internally inconsistent and miscompile even code that did not any clarification to be considered correct.