This property is no longer used and has now been marked as Deprecated.
Please refer to this information from the Release Notes to see what has changed:
New: Remote Mac/IOS compilation now uploads the entire workspace to the remote Mac and executes a separate remote instance of UBT rather than synchronizing individual actions.
This makes the remote compile codepath much simpler, and removes a lot of special cases that exist to support it previously.
The list of files to be transferred to the remote are listed as rsync filter rules in Engine/Build/Rsync/RsyncEngine.txt and RsyncProject.txt, which are applied to the root engine directory and project directory respectively.
Projects that need to customize which files are uploaded can add their own <ProjectDir>/Build/Rsync/RsyncProject.txt file, which will be included in the filter before the default version.
Attempting to package for iOS using Remote Build on Windows results in a failure if the project contains a plugin that uses "PublicAdditionalShadowFiles" in its Build.cs file. Log from the packaging attempt is attached.
This does not occur in 4.20
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-67666 in the post.