I was ruminating over a book I owned a while ago, TSR's Celts Campaign Sourcebook for AD&D 2nd ed. Since I no longer have the copy I used to own I found a new (to me) one to read through it.
I remembered there were fighting techniques in this book, but what I was surprised to relearn was that these techniques are labeled as feats (see Heroic Feats on page 28)! They cost proficiencies (weapon and non-weapon) instead of substituting for ASIs like in 5th edition, and they largely seem to be very specific techniques (such as catching and throwing a spear, or running along the yoke-pole on a horse-drawn chariot while it's in motion). While I didn't have every 2nd edition book, I had quite a few, and can't remember feats in any other resource I owned, though that may just be my (lack of) memory.
What are some of the first examples of feats in any edition of D&D? Is this campaign book one of the first times we see feats? Were there other 2nd edition books with feats? Did feats come even earlier in D&Ds history?