It isn’t restricted to Boost Mobile. It is only available on devices with the C1 or C1X modem, though. I assume this is because of specifics with the third party modems that most models in the wild have vs what Apple is doing in-house with their C1(X). If you call emergency services it will still provide precise location.
It could be a flag in the per-network CarrierConfig bundle. I imagine that would help with jurisdictions that might require this protocol for legislative reasons
