> Of course, if you're implementing POSIX vi, there are quite a few features that have aged poorly, like roff/troff macros and line-editing

What do you mean by "roff/troff macros"?

EDIT: Ah, you're probably talking about the "section-wise" movements, defined in POSIX with language like "A line whose first character is a <period> and whose second and third characters match a two-character pair in the 'sections' edit option (see ex)" - that's the first time I've stumbled upon the 'sections' and 'paragraphs' options in the Vim manual ... Very quaint!