Thanks.

> has been around since at least 1483

I should have guessed that it would have been used "before".

> what's truly bothering you...

I should say "Learnings sounds incorrect to me". Just the use of it makes me cringe. Just like nails across a blackboard.

Ironically, I do quite enjoy "obscure" English, in the sense that the US use 'fall' and 'soccer', both of which are "old" English, but fallen out of use in the UK.

When you write for a general educated audience, it's best to avoid words that will strike their ear as out-of-place, uneducated, or unexpected. You don't want readers to stumble over words or turns of phrase that sound ugly or unfamiliar to them. It distracts from the message.

From the perspective of descriptive linguistics, "learnings" may be acceptable, but descriptive linguistics isn't the only factor governing how writing is received. It should be avoided unless you're writing for an audience in which it's already in general use.

The issue isn't rules about forming nouns from present participles, but custom and usage in the general population and in that section of the population who read and write at an educated level. It's a matter of judgement and style. What is customary changes over time, but that doesn't mean it should be ignored.

As to your final point, many words were in use in 1483 (and 1943) that would be out of place today.