How do I remove IIS custom header using Powershell?

To remove the header on iis level:

Remove-WebConfigurationProperty -PSPath MACHINE/WEBROOT/APPHOST                                  -Filter system.webServer/httpProtocol/customHeaders                                 -Name .                                 -AtElement @{name='X-Powered-By'}

And for a specific site:

Remove-WebConfigurationProperty -PSPath 'MACHINE/WEBROOT/APPHOST/Default Web Site'                                -Filter system.webServer/httpProtocol/customHeaders                                -Name .                                -AtElement @{name='X-Powered-By'}

Adding a new custom field eg. xff-ip to have remote client ip from x-forwarded-for request header

Add-WebConfigurationProperty -pspath 'MACHINE/WEBROOT/APPHOST'  -filter "system.applicationHost/sites/siteDefaults/logFile/customFields" -name "." -value @{logFieldName='xff-ip';sourceName='X-FORWARDED-FOR';sourceType='RequestHeader'}

Or for a specific site:

Add-WebConfigurationProperty -pspath 'MACHINE/WEBROOT/APPHOST' -filter "system.applicationHost/sites/site[@name='My Super Site']/logFile/customFields"  -name "." -value @{logFieldName='xff-ip';sourceName='X-FORWARDED-FOR';sourceType='RequestHeader'}

Removing your added custom logging field eg.xff-ip

Remove-WebConfigurationProperty -pspath 'MACHINE/WEBROOT/APPHOST' -filter "system.applicationHost/sites/siteDefaults/logFile/customFields" -name "."  -AtElement @{logFieldName='xff-ip'}

Or from your site only

Remove-WebConfigurationProperty -pspath 'MACHINE/WEBROOT/APPHOST' -filter "system.applicationHost/sites/site[@name='My Super Site']/logFile/customFields"  -name "." -AtElement @{logFieldName='xff-ip'}