All supported versions of RT include RT::LocalPluginPath
Warn much more obviously on failure to find the expected directory
Remove RT3-specific code
Set a default lower bound of 4.0.0
Update documentation on file placement

The paths described are the paths before local/plugins/ existed.
Be more forgiving of entered path to RT.pm, stripping off final /lib if need be
Don't grow @INC with each failed guess at $ENV{RTHOME}
Remove misleading and useless defaults

On all supported RT versions (4.0.0 and up), the valid variables
(LocalLibPath, LocaltaticPath) are already defined in RT/Generated.pm.
The other variables are unused and unsupported.
Remove support for WITH_SUBDIRS

I have never seen this functionality used, and its premise (of
installing _part_ of a module) seems dubious at best.
Remove PREFIX support; it does not load RT.pm, and as such fails in _load_rt_handle
Unify %path and %subdirs logic
Enforce that RTx must be provided full extension name

Nearly 4 years is sufficient of a deprecation cycle.
etc can have locstrings to extract, as well
Add metadata to META.yml for minimum and maximum supported RT versions
Set a minimum perl version based on the minimum RT version
Rename Module::Install::RTx::Factory to a more appropriate name
No need to adjust @INC; callsites for RTxDatabase pass in appropriate -I values
$RT::SbinPath is set in all supported versions of RT
Remove references to non-existant variables

$RT::CORE_CONFIG_FILE and $RT::SITE_CONFIG_FILE were removed in
96411640, and thus this section of code has been a no-op since 3.7.1.

RT::LoadConfig will die if it cannot load RT_Config.pm, although the
message is somewhat misleading, as it refers to "the user/group your
webserver is running as" which is irrelevant in this context.
Intercept permission-denied messages and adjust them

While matching on the error using regexes is somewhat fragile, all
supported versions do currently use the same phrasing.  Attempting to
replicate the entire logic of RT::Config->LoadConfig in order to
re-check permissions seems like too high a cost for the rather low
chance that we re-word these messages in a stable series -- regardless,
the failure mode is no worse than at present: a misleading warning.