Blood donation and blood letting have other effects and confounding factors. You're supposed to be healthy to donate blood regularly, so right there you have an explanation for the observed longevity correlate. But it would also have other effects like reducing iron: people argue many men have too much iron later in life, while other donators like women may have too little, so that's mucking up your net effect.

Filtering blood, or diluting with saline, avoids these issues and helps get at the question of inflammatory & aging factors vs youthful factors.

I always thought this. There seem to be issues with a few things which the body has problems eliminating (heavy metals, new synthetic chemicals not found in nature, etc) Why not give your body a break by diluting all that a liter per month of blood at a time. I don’t imagine there would be a huge effect, but I do imagine a nonzero effect.

Indeed. On the other hand, with classic bloodletting, you're also losing red and white blood cells, unlike the plasma-only technique, which preserves them.

> Maybe there was something to the practice of leeching

Yes. Hirudine.