I was playing through Super Mario World, and I jumped, and right before I hit the ground, I pressed B, and i jumped again. Not sure if this is a old thing, a feature, or i just made a groundbreaking discovery.

There's a few things this could be, since your description is pretty vague. Namely: input delay, still-active flight meter, just.. you know... jumping twice

Sounds like input delay to me.

Even if this worked the way you think it did, that helps a speedrun how?

If buffering jumps was possible in this way this would make things like optimal climbing (mortons for example) a lot easier, anywhere you have to land and jump in a tight window really ... dp3 no cape jump on the on/off switch.. maybe even neutral fixed speed maintaining would become viable in more situations!

The game already has like 2 frames of natural input delay, so if you want to jump as soon as possible you need to press the button before you land. Any additional delay incurred from signal upscaling/conversion will magnify this effect.