there is really no open "good" supl server and that is the reason we do not set it by default and it is left to the user to enable. some carriers have some but they are mostly just a frontend for supl.google.com. here is a list of supl servers i know of but keep in mind, there is no open source server out there. these are all run by big tech companies
supl.attmex.mx:7275
location2.kddi.ne.jp:7275
wpde.nate.com:7275
supl2019.lbs.sprint.com:7275
supl.geo.t-mobile.com:7275
and last but not least the most functional server owned by google
supl.google.com:7275
i do not know if anything other than the google one is functional so you may need to do some research there.
@fakeshell Thank you very much! I will test out a few servers and I realize the privacy implications. I am not really sure how to know though if it works or not other then getting quicker location accuracy in PureMaps for example.
Is the port mandatory, or we can omit it and the default would be used?
I found one on some Raspi manuals and it did not error out or anything, but I am not sure whether it is actually in use. Maybe I can see it in some logs somewhere, will poke around.
supl.grapheneos.org Graphene OS still uses the Google service but Graphene OS strips the IMEI and phone number from their request before passing it on to Google.