I've been seeing this warning message when calling New-OSCustomizationNicMapping for a while now. No one seems to have commented on it anywhere online, which is odd.
I create my customization using New-OSCustomizationSpec and pass it to New-OSCustomizationNicMapping.
What is the proper thing to do here? Continue to ignore this nonsensical warning?
The warning is not really nonsensical.
It just says that you can only specify 1 value on the OSCustomizationSpec parameter of the New-OSCustomizationSPecNicMapping cmdlet.
What you are doing is completely valid, you just get the warning in any case.
If you don't want to see these kind of depreciation warnings, you can use the Set-PowerCLIConfiguration cmdlet with the DisplayDeprecationWarnings parameter set to $false.
Which a lot of users are doing is my impression.