In Stata 16, the output of help usersite tells us about the "g" lines in the package file.

g specifies that the file be installed only if the user's computer is of type platformname; otherwise, the file is ignored. The platform names are WIN64 (64-bit x86-64) for Windows; MACINTEL64 (64-bit Intel, GUI) and OSX.X8664 (64-bit Intel, console) for Mac; and LINUX64 (64-bit x86-64) for Unix.
And this is taken from page 1667 of the Stata Base Reference Manual Release 16.

If however we look at page 1675 of the Stata Base Reference Manual Release 14 we are told

g [...] specifies that the file be installed only if the user’s operating system is of type platformname; otherwise, the file is ignored. The platform names are WIN64A (64-bit x86-64) and WIN (32-bit x86) for Windows; MACINTEL64 (64-bit Intel, GUI), OSX.X8664 (64-bit Intel, console), MACINTEL (32-bit Intel, GUI), and OSX.X86 (32-bit Intel, console) for Mac; and LINUX64 (64-bit x86-64) and LINUX (32-bit x86) for Unix.
Here is the problem I encountered on a recent topic about the synth command. The output of ssc describe synth tells us that it will work with Stata 9.2 or later. The package includes a compiled plugin in versions for the various operating systems, and g lines control which one is installed.
Code:
g WIN32 synthopt.win32 synthopt.plugin
g WIN64 synthopt.win64  synthopt.plugin
g LINUX synthopt.linux32 synthopt.plugin
g LINUX64 synthopt.linux64 synthopt.plugin
g MACINTEL64 synthopt.mac synthopt.plugin
The problem arose when a user of Stata 14 for Windows installed the synth package. Their system reported either WIN64A or WIN, neither of which is accommodated within synth.pkg, which dates from July 2018. The end result is that no plugin is installed, and this failure happens quietly. The failure is discovered when synth.ado in its final line executes program synth, plugin and Stata reports that the file synth.plugin is not found.

My guess is that at some point the developers updated the synth.pkg file to accomodate the Stata 15 platform names, but neglected to leave in place the g lines for the older platform names. As a consequence the code can run in Stata 14 but cannot be installed without a workaround.

I want to report this to the developers and would like to be as helpful as possible. I can keep going backward in the PDFs that are available at stata.com, but perhaps someone has a definitive list of the historical platform names and the Stata versions to which they applied.