All this discussion is making me incredibly glad I posed the question, as it does indeed seem that before num and nl, line numbering was more of a "to each their own" rather than there being a canonical best practice.

By the way, what inspired the whole quandary in the first place was doing some editing on a V6 UNIX system and realizing I hadn't the foggiest idea of how to get the system to spit back line numbers to remind myself what addresses to supply to various ed commands.  I suppose at the time using a context search was such an effective means of getting ed's cursor where you wanted it that having a means to number lines wasn't crucial to effectively working in ed, so long as one was good with their regex and searching.

I'm realizing I might need to reach out to the maintainer of the nl manpage in FreeBSD (and perhaps other OSs and distros) as the history section of the page posits that nl was first introduced in SVR2, when in reality the earliest public offering was System III, although the trail goes cold at that point, I can only assume it came from USG.