I don't understand.

These days if they only did stats after the 7th inning only relievers would be included.

Good one!

Actually, a set of stats I'd like to see is OBP, SLG, and OPS allowed for pitchers.
This would be more useful than WHIP, which is a kind of version of OBP, only WHIP is not as precise or intuitive. And what WHIP leaves out, most importantly, is SLG. A pitcher could have a low WHIP but be giving up too many doubles, triples and HR. This would lead to a higher ERA. And we'd know why if we had pitcher SLG.

Baseball-reference.com does in fact provide pitcher OPS allowed. Just click on "More Pitching" over a pitcher's main stats, scroll down, and you'll find "Batting Against --Pitching." A couple of spot checks indicate that Greg Maddux's career OPS allowed was .649 vs a MLB average of .745. Not bad.

But Mariano Rivera's OPS allowed was even more amazing, .555 vs a MLB average OPS of .752. That's just another measure of Mo's excellence, the like of which we may never see again.

Anyway, SOM has all the stats to calculate pitcher OPS allowed, but we'll probably never see that, either.