beaconDB is a drop-in replacement for MLS, which uses the same format request that’s used by Mozilla’s Ichnaea.
The source code is available on Codeberg: https://codeberg.org/beacondb/beacondb
You can contribute to the project by using an app like NeoStumbler (GitHub) or Tower Collector (GitHub) to submit location reports. NeoStumbler does Wi-Fi, Bluetooth and GSM, while Tower Collector can only do GSM cell towers. Both are FOSS and available on F-Droid.
It is also recommended by the GrapheneOS project: https://grapheneos.social/@GrapheneOS/112759509558471713
https://grapheneos.org/articles/positon-location-service
Just keep in mind that it’s still in relatively early development, which is why it really needs contributions.
It really is as private as it can be, and the developer is really dedicated to improve user privacy. Other solutions (SUPL, PSDS) aren’t much better in terms of privacy. And it’s definitely not a solution in search of a problem, as pure GNSS is really slow and consumes a lot of battery. You can try this out yourself if you use GrapheneOS, by going into the location settings and disabling both SUPL and PSDS. I tried it, and I can tell you that it’s bad (in my opinion unusable).
