You certainly shouldn't donate blood in unsanitary conditions and while you are ill and weakened from some other sickness. Which is the conditions under which bloodletting usually occured in the past.

I’m not saying people should do it, of course. Just wondering if there’s at least a grain of truth in it.

I'm sure there's a grain of truth to it, else it wouldn't have been pushed as a treatment; likewise, there's a grain of truth in using ivermectin to treat the 'rona (in that it increases the survival chance of people who also have intestinal worms, see https://astralcodexten.substack.com/p/ivermectin-much-more-t...)

Any usage prior to the widespread adoption of the scientific method is largely irrelevant.