Unless you’re on the phone for most of the day/night and need to use Bluetooth devices constantly, there’s no reason to keep Bluetooth on.
Keep Bluetooth OFF if you don’t use it. Only keep it ON briefly if you have to use it (AirDrop, wireless speaker, etc…) otherwise, it’s an open invitation.
In a vastly connected world of Internet-connected devices, you don’t want your devices to be an open-door.
Exploitation devices evolve very fast likely at the same rate as new domain names being registered, parked then used for nefarious purposes.