But it doesn’t seem to make sense to compare Feller’s k rate against modern pitchers. The real question is:
How did he perform against others in his era during his peak years?

He simply led the league in k/9 for 5 seasons including 4 seasons in a row before he entered the military. He also led the majors in H/9 3 years in a row. Feller was a beast and the most dominant power arm of his generation and the numbers back this up. In his era, pitchers pitched to contact, and power wasn’t stressed nearly as much. When you have guys not swinging for the fences it is easy to see why pitchers were not striking out batters at today’s rate. It isn’t fair to use today’s standards against a player of a different era.

This is attributed 100% to two main things:

1- pacing: pitchers in that era knew they would have to throw a complete game virtually every time out so they paced themselves, bearing down and showing their best stuff only when the situation called for it. Today, when the #8 or #9 hitter is up the pitcher looks for a strikeout, while in those days they pitched to contact to save bullets because they were going to throw 130+ pitches

2- Batters strikeout much more frequently because change of strategy. In those days a strikeout was an embarrasment to a batter and they were often taught to “choke up” with two strikes. Today, with sabermetrics, the idea is an out is an out whether a K or groundout, and batters are told to still swing for the fences even with two strikes