This question is not a duplicate of:
They are from a time when Stack Exchange (SE) used Prettify for highlighting which didn't support PowerShell. SE now uses highlight.js which does. It now comes down to:
open a question tagged
[feature-request]
to add syntax highlighting for a language which is supported by the highlight.js project, but it is not supported by the version of highlight.js used on Stack Exchange.
Can Stack please use a version of highlight.js that supports PowerShell?