For those curious, in Windows you can still use xp_cmdshell and run fsutil volume diskfree <driveletter> where "<driveletter>" is the drive letter name such as "fsutil volume diskfree C:".
a lot shorter in Linux with df. df -h for "human readable" sizes (KB, MB, GB, TB, etc) rather than bytes.
Without xp_cmdshell, I am not sure how you'd manage that though.
I do feel like SQL Server is the wrong tool for monitoring free disk space though... If SQL Server is the hammer, not every problem is going to be a nail...