That's quite arbitrary.
In my opinion, there have not been many PC-compatibles at all, if any. Take 8088 MPH for example. So far only a handful of clones can run it properly.
Sort of, sure. I've never seen any sort of "official" test suite to determine compatibility. Even if one were to go only by a minimum of IBM's officially documented specs, things changed as they moved to the IBM AT, PS/2, etc.
But during the 80s and 90s there was a generally accepted definition of "IBM PC Compatible", in which one could reasonably assume PC-DOS and their business software would run as-is if their vendor advertised as such. If it didn't, then there was something wrong with the hardware. If anything deviated too much - including some of IBM's hardware - the market would respond negatively.
When Apple moved to the x86 platform, it shared many features of common "PCs", yet they removed enough compatibility that no one really argued they were now officially "PCs".
Which is really the same point as the OP. There is NOTHING IBM PC-ish about these newer computers. They removed ISA, PS/2 mouse ports, floppy controllers, serial ports, lpt ports (everything useful), and most CPUs spend their time running in a foreign 64-bit mode. The last holdout was BIOS compatiblity, mainly needed for some special kinds of boot CDs and perhaps some PCI devices.
Furthermore, if they lock down their Secure[from competition]Boot so no one can even try to boot others OSes, then it no longer even deserves the general title of "Personal Computer".