At least 19.1. Not tested yet on others.
A PowerShell Script can be assigned to a policy so that it applies to all applications, as indicated here:
And as documented here:
Once this script is selected and you press -Save-, the setting will become locked for all sub-folders and apps within this folder. It can not be unlocked or over-ridden unless the PowerShell script is de-selected.
This is by design, due to the "extra fields"... If we allowed the PowerShell Script setting to be modified by child policies, any "extra field" values that were set by policy at higher levels would lose their context and likely be invalid.
Resolution / Work Around:
It is recommended that separate policies be used for this, and that PowerShell scripts be applied as far down the "tree" or nodes as possible.