I’ve read at least several hundred books in English, and probably hundreds of thousands of news stories and blog posts, and don’t recall encountering it before. It seems to appear mostly in searches for its definition and gre prep...

Try, say, Google ngram. Would you say that, for example, “disharmonious” is a rare word? Graph them together. Pulchritude is not even moderately rare.

I’m not sure what you’re trying to say here; pulchritude twice as rare as disharmonious, which is relatively rare. It’s an order of magnitude rarer than effervescent, as an example, which is a word I would consider uncommon but in most people’s vocabularies. And given how much more common pulchritude is - surely physical beauty is one of the most common subjects in any language - while disharmony and effervescence are more complex and rarer in practice, I think it speaks to how little the word is used. Its synonym, beauty, is used 2000x as often.

Except that pulchritude beats disharmonious for over a hundred years. And even now, it’s half as common as disharmonious, which is a commonplace word. But if you think that a word like “disharmonious” is in any sense rare, then I don’t think I can convince you with data.

When compared to "harmony" usage of the other words is basically zero. Harmony is a common word, and it is the root which makes "disharmonious" easy to understand despite its extreme rarity.

Even "disharmony" is far more common than "disharmonious."

Disharmonious just seems common because it sounds similar to a word we're familiar with. Most times I can recall, a writer would craft a sentence using the word 'disharmony' instead, which is many times more common than either. What led you to pick disharmonious as a comparison point, anyway?

I agree. I think if you're going to use "disharmonious" as your benchmark, you should be comparing it with "pulchritudinous" instead.

Probably learned it while prepping for their SATs

If you search for the exact phrase you'd see it is fairly commonplace in literature.

It is a pretty rare word to come across in modern writing.

"And a sea of pulchritudinous models would have served as a lesson on how not to choose words: the ugly pulchritude sounds like the opposite of what it means, and it is one of those words that no one ever uses unless they are trying to show off.” https://www.goodreads.com/work/quotes/40167209-the-sense-of-...

A Redditer notes that a similar word is common in Spanish. For speakers of Latin-derived languages (French, Spanish, and of course Italian), English vocabulary quizzes are generally a bit more easy because English took its literary words from old French and Latin. Thanks William The Conqueror for that.

That said, as a french speaker I would have had no idea what this word means if I did not took Latin in high-school.

It's not common at all.