Yep, I knew that
[Raak] That's it, in essentials, though not quite quite. Quoting my handbook in etiquette from 1933:
"Although the use of the address ni is more common today than ten or fifteen years ago, it is not accepted in ordinary social life. [...] it is however permitted to include a ni here and there in conversation, provided that one also includes the title. Thus, it is possible to say "Oh, so the countess was in Visby last summer, how did ni find it?" or "Is the consul going to Paris, weren't ni there last spring?" But you cannot be too young if you are to use ni to older people [etc etc for 30 more pages about use of titles I kid you not]"
It does say that using "ni" to colleagues is fine, though, as well as some other equal-status situations where it was OK. There's a whole chapter about when to stop using titles -- always a mutual process, you stopped using each other's titles and said du (sing. "you") instead of ni. A lot happened in the decades following with an increase in "ni" and decrease of titles, but the big big revolution was in the late 1960s, when the head of a government office declared that he would say "du" to all his employees and expected "du" back. (Well, that's it in a nutshell anyway -- it was more complicated of course.)
Anyway, the funny thing is that after all that, today Swedish uses virtually no titles at all. We have "Mr", Mrs", "Miss" and "Dr" and various old nobility and royal titles, but apart from royalty nobody uses any titles, ever. (Very young schoolkids tend to call their teacher "miss" or "teacher", but once they hit 10 or so it's strictly first names.) So when buying, say, plane tickets from British Airways, a Swede is struck by the necessity to put a title there -- which feels very very foreign.