2016 Bonneville T100 Battery Isssues

Sounds like a bad battery. 4 years out of a battery is expected. Any battery that is not fully charged all the time gets damaged over time which is normal. That is why it is good to keep a battery on a battery tender. I always plug in my tender whenever I am not riding my bike. My batteries have always lasted many years. Or it might be a bad voltage regulator.

 

Sounds the USB charger cable is at fault take it off or disconnect it and see what happens to the battery which should be reading at least 12.5 volts

 

don't like USB charger leads get a separate one to attach direct to battery

 

USB chargers will still draw some current, even when there is no USB device connected (it contains a 12V to 5V converter circuit) - although with a device connected it could easily be pulling several amps
And T100 has a pretty small battery
Unless you absolutely HAVE to have ability to have it be live with ignition off, it would be better to have it connected to an ignition-switched circuit.
When you get your new one however, if you have a multimeter available, you can always test the current it draws with no USB device connected (connect the negative, set multimeter to read current and connect the probes between the battery positive and the USB positive terminal; be sure to only connect like this and remember to switch back to volts - if you connect meter to a voltage source while on current, you will blow the meter fuse)

 

You can use the same circuit that supplies your Horn/Turn Signals/Brake Lights
That would be the Orange/Green circuit which will only be active with ignition on and have plenty of current available.
You can tap that on the main harness side of the connector that feeds the right bar switch inside the headlight bucket, which might be a convenient place for a front end mounting location - or the wire that goes to the horn itself, or the one that goes to the Turn Signal Relay or to the REAR Brake Switch (NOT the light - has to be on the Orange/Green at the switch)