Rather, human typesetters of professionally printed material have always (well, since it was invented) used the em-dash. Handwritten dashes rarely clearly break down into categories that are clearly exactly matching one or the other typographical dash, and until relatively recently with proportional display fonts with large character sets and fancy input methods, typed (whether on typewriter or computer) text was unlikely to directly contain typographical dashes, though some systems (especially publishing/typesetting toolchains!) had system-specific, ad hoc means of representing them.

OTOH, as long as user-interactive web content has existed—so “always” in a context of a particular view of the online world—em-dashes have been part of it, because the facilities that make it easy to use (whether automatic replacement, or various keyboard input modifying mechanisms) have been sufficiently common that a robust minority of users have regularly used one or more of them.