By code the junction box must be accessible, if you read the definition of accessible as applied to wiring methods “capable of being removed or exposed without damaging the building structure or finish or not permanently closed in by the structure or finish of the building”.
So wiring methods located behind removable panels designed to allow access are not considered permanently enclosed and are considered exposed as applies to wiring methods.
What is this saying? You can make a fancy cover to hide the junction box and save it for later use. I have been doing this for years using Velcro strips to hold my little picture frames in place, one day an inspector asked me to open one as he liked my little picture frames in a basement that allowed access to the ugly 4x4 j box cover(s). I pulled a bit two quickly and broke 1 side before the Velcro released, he said I could screw them in and did not have to be no tools (as I had thought for years) but as long as the finish was not messed up on the wall I could use screws.
I still use Velcro except with very large covers old habits you know ,, but as long as the box is accessible you don’t have to remove it. If you want to close in the wall and abandon it the wires do need to be isolated from power and tagged or removed.
If it were me I would keep it accessible and cap the wires put the cover on and make a frame or hide it behind the mirror as a possibility for future use.