Are SPNs required on the service account for the GPOAdmin tool or what functionality is removed if the SPNs are removed?
The SPN is required. It is used by Kerberos authentication to associate a service instance with a service login account.
If you were to remove the SPN It wouldn't work at all, as the service authentication wouldn't be able to work. the process is described in the MS document below if you remove the SPN all the processes will be broken.
https://learn.microsoft.com/en-us/windows/win32/ad/service-principal-names
