I would have hooked the smartphone to a small solar panel. The natural daylight cycle would have made sure that the smartphone kept having charging and discharging cycles.
I doubt the traffic hitting it would be sufficient to drain the battery overnight.
That's silly. Phones and laptops absolutely have charge controllers, and can manage with a phone plugged in all the time.
The fact that they can doesn't mean they will.
On older devices the controller might make some assumptions that holds true with a new battery, but very much doesn't with an old and worn one.
My Macs have all been sensible about it, but I've seen Windows machines with batteries that just died from being plugged in all the time not even 10 years ago. Even if that specific instance was just a bad battery and not due to a charge controller, I have no faith in Random Windows or Android OEM Number 582 doing this correctly.
For devices that are fixed, I'd prefer to eliminate the potential of there even being a problem in the first place.
I have multiple devices with lithium batteries plugged in 24/7. A kindle that I use as a display for example. So far nothing exploded. If exploding kindles were a thing I guess I would have heard.
I have not had anything explode, but I have had Macbook batteries expand on me on two separate occasions to the point where the case was visibly warped. Both times I was away from home, so it was extremely inconvenient.
I've gone through a dozen or so LiPo-utilizing portable devices at my property in the Mojave desert. All it takes is a single season for many of these batteries to swell up to such an extent the enclosures split open.
Ostensibly they contain charge controllers and temperature sensors, yet they're unable to prevent this outcome when the ambient temperature exceeds 110F day after day while the device stays on in a hot attic w/usb-c pd connected.
Fortunately I haven't had any burst into flames yet, but after a few years of seeing this pattern repeatedly I stopped deploying anything containing LiPo batteries at the property.
YMMV - but IMHO it's prudent to exclude these batteries from such unattended, powered 24x7 devices.
For how long?
I have a fire tablet I hucked into my wall to use as a home assistant console.
It's been in there for 5 years now, always plugged in.
Until the heat death of the universe, and a couple beyond.