TL;DR contemporary writers use ‘above’ and ‘below’ for intratextual referencing—how long has this been the case, and did this usage coincide with the introduction of PC publishing software?
I have noticed a number of paper and book authors referring to previous or upcoming parts of their work using the terms ‘above’ and ‘below’, rather than only using those terms for reference within the current page. In my (anecdotal) experience this usage seems to be the norm in contemporary writing.
It seems to me that it would have made little sense back when works were hand-composed into books, where a physical reference like ‘overleaf’, a temporal reference like ‘previously’ or an absolute reference to ‘§ whatever’ would be more intuitive. However, it makes sense conceptually when you consider how a document-in-writing is presented by default in desktop publishing software and LaTeX: as a continuous series of pages aligned top-to-bottom, or as a continual document of markup.
My theory, then, is that this usage of document-wide ‘above’ and ‘below’ references will coincide with the introduction of dekstop publishing and typesetting tools in the 70s/80s, and that before this point these would only have been used to refer to things within a given page. I am not familiar enough with Google Ngrams to determine this myself though.