Is xp_create_subdir supported on Linux SQL Server Some system extended procedures work on 2017 RC2

e.g.

exec xp_dirtree @directory = '/var', @depth = 1


Correctly returns the directories contained in /var

However:

exec xp_create_subdir '/foo'


Returns

Msg 22048, Level 15, State 0, Line 18
Error executing extended stored procedure: Invalid Parameter


Is the procedure supported, and if so what is the syntax for the parameter(s)?

Tested running on Ubuntu, inside Docker container on MacOS.

select @@VERSION
Microsoft SQL Server 2017 (RC2) - 14.0.900.75 (X64)   Jul 27 2017 08:53:49   Copyright (C) 2017 Microsoft Corporation  Developer Edition (64-bit) on Linux (Ubuntu 16.04.2 LTS)
According to the latest official release notes, no it is not supported.

The following features and services are not available on Linux at this time. The support of these features will be increasingly enabled during the monthly updates cadence of the preview program.
...
System extended stored procedures (XP_CMDSHELL, etc.)

It does say that there will be increasingly available but does not make any commitment toward a time or viability at release.