Apologies if this has been asked - I did spend a bunch of time searching.
In the unlikely case that a powershell question is specific to Windows Server, the "windows-server-powershell" tag works fine, as it's very narrow/specific.
However, it's more likely that a powershell question could apply to any Windows version - and even other OSes that can run powershell, eg Linux!
It would make sense to tag a question with multiple generic tags instead, eg "Windows-Server" and "PowerShell". Stack Exchange knows how it's done:
Hope that makes sense