Maybe I missed declining a prompt in setup, but this latest feature was opt-out, automatically enabled after a software update, and only notified me once, the first time I powered off the phone after updating. To disable the feature, I had to go four levels deep in Settings, with the last two levels having seemingly unrelated names, disable a toggle, and accept a misleading confirmation prompt.

It jumps off of general Find My functionality, so pretty much anyone that has BT enabled, and doesn't want their phone to be sellable when stolen, will act as a proxy node.