And we could call the PPro the first "modern" CPU, seeing as it has far more in common (with it's micro-op translating RISC core, i686 instructions, focus on 32-bit rather than 16-bit, etc.) with newer processors than the Pentium does.
Personally I think that the PPro is a bit of a special case, however, being the "first" of these CPUs, and that it's really the exact point where vintage stops and modern begins. The focus on 32-bit optimization really screams "lack of backward compatibility!" when DOS was so prevalent, making the PPro a CPU that in some configurations (Dual PPro with P2 overdrives) could even run a modern OS like XP or perhaps even 7.
Personally I don't have any built PPro machines and don't have a strong opinion as to whether or not they should be off topic or allowed, but I do feel that they provide a nice clear point at which to divide vintage and modern - so either the PPro should be off topic, or it should be a special case with the Pentium section being noted as Pentium/PPro like there was a 386/486 and 808x/286 section before them.
This would make P2 the earliest thing in the Off-Topic section and provide a hard line for vintage->modern with Pentium sitting behind as "Classic" or "Transitional", discussable here for a combination of it's legitimate claims as a vintage machine, and for it's usefulness in bridging modern and vintage machines.