Per this twitt:
https://twitter.com/GrapheneOS/status/1601367470639808512?s=20&t=IufHte4n7iTmSFujRlox0Q
"Default SUPL server is http://supl.google.com but carriers can choose the implementation. If you don't want SUPL being used, you simply have to delete supl from your APN types."
and
"Yes, that disables SUPL, which is either using the default http://supl.google.com or a carrier provided SUPL configuration. Carriers aren't providing their own SUPL servers in practice since control plane A-GNSS exists. You may also want to remove dun"
Changed "APN type" from:
default,supl,dun
to:
default