Email from Brian D. Ripley, Emeritus Professor of Applied Statistics, University of Oxford
The future of OpenGL is uncertain (except on macOS, where it has no
future). So it seems reasonable to require rgl only when essential to
the package. These packages have it in Depends/Imports but seem not to
actually call it in their checks (established using a fake install). It
is possible that the sole purpose of the package might be to do
interactive visualizations which are not checked, but that seems not to
be the case here. We noticed calls to rgl functions in \dontrun{}, but
they would better be conditioned by if(interactive()) (see 'Writing R
Extensions').
Please move rgl to Suggests and use conditionally (see §1.1.3.1 of
Writing R Extensions') at the next package update.
Email from Brian D. Ripley, Emeritus Professor of Applied Statistics, University of Oxford