As has been explained in the former rgdal vignette https://r-spatial.github.io/evolution/CRS_projections_transformations.html,
now posted on the Evolution project site, coordinate reference systems
are represented differently when PROJ < 6 and PROJ >= 6.
To alert interactive users to the possibility that their workflows
might be degraded unless they take the representational changes into
account, the sp proj4string() method
issues warnings if a "CRS" object does not have an
accompanying WKT2 2019 representation (stored in a
comment attached to the "CRS" object). This is
because proj4string() only returns the PROJ 4
representation, not the accompanying WKT2 2019
representation if any, so may constitute a loss of information. See also
https://stat.ethz.ch/pipermail/r-sig-geo/2020-May/028125.html
for further discussion.
In general, it is good practice to replace all use in packages of
proj4string() with slot(, "proj4string") and
its assignment form by the assignment form of slot(); when
sp::proj4string() is not called, many of the warnings are
suppressed anyway.
Both CRAN checks and use of checks by end users may trigger spurious
errors, if the versions of PROJ and GDAL differ from those on the
development platform. Since the WKT2 2019 representation is
only generated on systems with PROJ >= 6 (and GDAL >= 3), warnings
should not be expected when the same code is run on platforms with older
versions of PROJ and GDAL. There are many such platforms, including RHEL
and CentOS systems, as well as Ubuntu 18 and others. Think of a large
lab with a cluster runnning such a system version, and that those
installing software may wish to run R CMD check or just
your test suite to confirm that things are as they should be. They will
be dismayed to find that tests fail, but will not be able to find out
whether this matters for them.
Consequently, it is best either not to check warnings that perform differently when run under different versions of upstream external software (PROJ and GDAL), or to condition on the software versions with different expectations depending on the version.
Fine-grained control may be obtained using
sf::sf_extSoftVersion() or terra::gdal() to
determine versions if necessary.