Patent infringement threat(Skyhook Holdings, Inc), lack of further investment seems to have been the reason of killing the service around 2021. It seems google location service seems to be the only player in town(other than OpenCellID, which I do not have experience with).
MLS never had an open source WiFi database. Only the phone cells were public.
My interest was more on the point that their service was/is used by so many open source projects. Would be interesting to see what alternatives they replace it with.
Years ago, there was a UnifiedNLP back-end for Android that used Apple’s service: https://github.com/microg/AppleWifiNlpBackend
Back then it worked fine. No idea about today. That repo isn’t maintained any longer.
It’s working.