Extinct = none left
That's a change, then. Our working definition when I was with the MUL team was "Extinct" = "Not enough left to be notable." The
Mackie was extinct after the Amaris Civil War, but several were left around; I seem to recall two in the Clan Homeworlds and at least one in the Inner Sphere. The
Phoenix Hawk Mk I is extinct, but similarly there is one in a museum. The
Gladiator was extinct for centuries, but one was operating during the early days of the Jihad, before it was put back into limited production. The
Shogun is effectively extinct during the Jihad, but we know of a bare handful operated by Wolf's Dragoons and Wannamaker's Widowmakers. Hell, the
Shadow Hawk LAM - which was one of the very few units in recent history to be given hard (and extremely low) production numbers - might even turn up, given that one chassis disappeared.
There are only two "rarities" the MUL is concerned with. The first is "Unique," and denotes a singular name of a certain unit. The other is "Extinct," as discussed above. Neither means "only one" or "none left," however! While it's true that there is only one 'Mech named
Yen-lo-Wang, its configuration could have been duplicated numerous times; indeed,
Centurions armed with AC/20s are common enough to be mentioned in TR3025. "Unique" configurations of OmniMechs, like
Daishi Widowmaker, can be - and are - used by anyone with the appropriate pods.
Best of all, you can ignore all of the above and do whatever you want in your own games. The MUL - like all of "canon" - is a guideline, not a straightjacket.