The current warning text reads:
"WARNING: Parameter 'OSCustomizationSpec' is obsolete. This parameter no longer accepts multiple values."
However, the parameter is not obsolete.
The correct text for this warning is:
"WARNING: Parameter 'OSCustomizationSpec' no longer accepts multiple values."
Are you referring to New-OSCustomizationNicMapping cmdlet? This parameter has been changed not to accept multiple values back in 2017 and this warning message no longer exists any more.