> This behavior is inconsistent with `os.path.abspath` where it always 
> returns lowercased drive letter, 

ntpath.abspath calls GetFullPathNameW, which generally preserves the input case of the device/drive component. But it doesn't always preserve drive-letter case. In particular, a drive-relative path gets resolved with the upper-case drive letter:

    >>> os.path.abspath('c:spam')
    'C:\\Temp\\spam'

That's a corner case that maybe needs to be addressed for ntpath.abspath. However, regarding ntpath.realpath, I see no reason for it to preserve the input case. To the contrary, it should always make a best effort to normalize the input path to use the real device and component names and replace 8.3 short names with long names.

> and also causes a failure in Gecko build script: 
> https://bugzilla.mozilla.org/show_bug.cgi?id=1628726

IMO, the bug there is using a case-insensitive path as a key without first case folding the string.