https://powertoolsadapters.com/ specializes in exactly this.

For some reason they insist in shoving some USB PSU in many of those adapters.

That will drain the battery by itself over time. So you then must make sure to remove the battery from the adapter as per instructions.

That is just so dumb, it's beyond my understanding.

There's no reason a USB PSU couldn't have such a low quiescent current that you wouldn't notice it when there's no usb plug. No idea if they've done that there though.

Cheap design. You can do a shutdown and then have a controller go into deep sleep with picowatts of standby power consumption. A load connected wakes it up.