Hmmm - on the topic of salt.... late 19th century French coffee, had a secret ingredient.... butter.
One does wonder if perhaps that butter was salted rather than sweet?
A small dab of butter would give richness (being basically concentrated cream!) - and if the salt has an impact on bitterness...
Earlier on, Voltaire was famous for drinking 20-30 demitasse a day when writing Candide, so anything goes with the French. I think coffee prep in those days resulted in something we might find unpalatable.
Butter in coffee sounds like something you'd drink halfway up Everest. So yak butter and Himalayan salt. I'm looking forward to the taste-test notes.