> "LOUD, SHOUTING HEADINGS" are ultimately a relic from > the (tele)typewriter era.
It's just a technical limitation of teletypes that they usually could not deal with different font sizes. For the actual printed manuals, the man macros traditionally had the section headings printed one point size smaller, to lessen that impression of "shouting". This tells us that the capitalization was intended as a presentational effect. (But it's not clear why they did not attempt to achieve this automatically with .tr.)
