I don't know this but wonder if it is for the tow hook storage. I don't have one so don't quote me on this. From what I've been able to read they discontinued the hook in mid-2020. The tow hook cover is still on the front bumper but the hole or bracket that was there to screw the hook in no longer exists.
They also discontinued the rear air diffuser/sound cover in mid-2020 on the STs. I've heard some say that leaving the sound cover off has lead to a buffeting sound under the vehicle. This cover also lends support to the rear bumper cover, I've noticed this to be a bit floppy when I installed my sway bar. I've seen others complain of this but haven't heard of a real fix.