Is ALTREP "non-API"?

ALTREP is part of the official R api, as illustrated by the presence of
src/include/R_ext/Altrep.h. Everything declared in the header files in

that

directory is official API AFAIK (and I believe that is more definitive

than

the manuals).

That is not true at all - the presence of header does not constitute

declaration of something as the R API. There are cases where internal
functions are in the headers for historical or other reasons since the
headers are used both for the internal implementation and packages. That's
why this is in R-exts under "The R API: entry points for C code":

If I understand your point correctly, does this mean that

Rf_allocVector() is not part of the "official" R API? It does not appear to
be documented in the "The R API: entry points for C code" section.

It does, obviously:
https://cran.r-project.org/doc/manuals/R-exts.html#Allocating-storage-1

For many purposes it is sufficient to allocate R objects and manipulate

few allocXxx functions defined in Rinternals.h?you may want to explore