[Rd] Is ALTREP "non-API"?

[Hiroaki Yutani]

Writing R Extension[1] defines "API" as:

    Entry points which are documented in this manual and declared in an
installed header file. These can be used in distributed packages and will
only be changed after deprecation.

But, the document (WRE) doesn't have even a single mention of ALTREP, the
term "ALTREP" itself or any entry points related to ALTREP. Does this mean,
despite the widespread use of it on R packages including CRAN ones, ALTREP
is not the API and accordingly using it in distributed packages is
considered illegal?

Best,
Yutani

[1]: https://cran.r-project.org/doc/manuals/r-release/R-exts.html#The-R-API

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