Hmmmm… I use a Plantronics Voyager Edge headset--must to use the phone because of hearing impairment--and that model reports battery usage to iOS; if the Bluetooth indicator goes, I wonder if that means I'll lose the battery indicator too. Additionally, to answer the question, because of myriad iOS Bluetooth problems mostly involving A2DP usage by mono headsets, I have disabled A2DP in my headset and need/use the Bluetooth indicator to show that my headset is connected. (In other words, did I turn my headset off and forget about that?)
Perhaps I need to start doing some testing with my headset and iOS 12. I rue doing so, because of the sheer number of bug reports I've had to file against the Bluetooth implementations for the past several years. I'm rather operating on the idea that if I just ignore it all, Apple might accidentally "get it right". Heh, not bloody likely, but it is less anxiety-inducing.