Create some output with non-ASCII characters, e.g. this on German or Fresh Windows:

res <- processx::run("systeminfo", c("/FO", "csv"), encoding = "windows-1252")$stdout
substr(res, 2200, 2300)
#> [1] "5d4\",\"Es wurde ein Hypervisor erkannt. Features, die f\u0081r Hyper-V erforderlich sind, werden nicht ange"

So the \u0081 is not converted, apparently, even though that seems to be the default encoding:

❯ [System.Text.Encoding]::Default


IsSingleByte      : True
BodyName          : iso-8859-1
EncodingName      : Westeuropäisch (Windows)
HeaderName        : Windows-1252
WebName           : Windows-1252
WindowsCodePage   : 1252
IsBrowserDisplay  : True
IsBrowserSave     : True
IsMailNewsDisplay : True
IsMailNewsSave    : True
EncoderFallback   : System.Text.InternalEncoderBestFitFallback
DecoderFallback   : System.Text.InternalDecoderBestFitFallback
IsReadOnly        : True
CodePage          : 1252

This might be some systeminfo or Windows thing, because according to https://en.wikipedia.org/wiki/Windows-1252 \x81 should be unused:

According to the information on Microsoft's and the Unicode Consortium's websites, positions 81, 8D, 8F, 90, and 9D are unused; however, the Windows API MultiByteToWideChar maps these to the corresponding C1 control codes. The "best fit" mapping documents this behavior, too.[15]

However, 850 seems to work well:

res <- processx::run("systeminfo", c("/FO", "csv"), encoding = "850")$stdout
substr(res, 2200, 2300)
#> [1] "5d4\",\"Es wurde ein Hypervisor erkannt. Features, die für Hyper-V erforderlich sind, werden nicht ange"