Aha, interesting. I was doing some quick scanning on Google-Books and other places to try to find the earliest usage, using variants of "numeric literal" and "underscore" as search terms, and couldn't find anything pre-1990s. Now I find the Ada stuff, and it wasn't coming up because all the '80s Ada documentation/discussion refers to them as underline characters, rather than underscore.
(Incidentally, one of the perils of trying to draw historical conclusions from text searches, or data sets like Google Ngrams: you could get completely wrong results if you miss a shift in spelling or word usage.)
(Incidentally, one of the perils of trying to draw historical conclusions from text searches, or data sets like Google Ngrams: you could get completely wrong results if you miss a shift in spelling or word usage.)