Surprised a car that young would have a faded / dull boot lid.

I’m old school. T-cut and polish

 

Thanks Craig, it's actually the black spoiler that's faded.

 

I'm fairly sure that part is just matte plastic, by which I mean unpainted black trim, although it has been a while since I've looked at one in detail.
If it is a matte finish, DO NOT use any polishing or cutting compounds such as T-Cut, these will cut into the matte finish making it shiny.

I was going to suggest using a plastic trim restorers, however before doing that you should do this instead.

Spray on a 50-50 mix of isopropyl alcohol and water, give the product some time to work, 10-15 seconds, then wipe it off with a soft cloth, usually once wiped you can tell if there has been a change in appearance, however check after about 5 minutes to see if the dull effect has been removed.

If the above doesn't have the desired effect try a mild solvent, such as tar and glue remover, leave a couple of seconds and then wipe or wash it off.
I personally like CarPro Tar X, as this is plastic safe:

CARPRO Tar X - Tar & Adhesive Remover

Reduce intensive claying on your car, with Carpro Tar X pre wash tar & adhesive remover | Free Shipping

www.slimsdetailing.co.uk

IMPORTANT: Some Tar and Glue removers will eat plastic, make sure you use a plastic safe one.

Alternate option - ditch the stock OE spoiler and buy the Battle Garage racing carbon wing upgrade kit:

BGRS BRZ 86 Wing Riser Carbon Wing Full Upgrade Kit

-------------------------------------------------------------- DOWNLOAD INSTALLATION INSTRUCTIONS HERE -------------------------------------------------------------- OVERVIEW Upgrade your ZN6 with the BGRS Wing Riser Adapter Kit, designed to offer versatility and style whether you're running the...

shop.battlegarage-rs.com

 

Going by the weight of mine... I think the facelift spoiler is metal and based on funky patten in the coating (on mine at least) I think it's powered coated?

Either way I don't think it's classic paint, which could explain the fading, should someone have tired to give it a polish with traditional products before.

Whatever you decide to give a go, try it out on a well hidden area first (somewhere on the underside).
If that doesn't work out you could wrap it, paint it or as Will has suggested, grab the kit from BGRS.

I've just installed the BGRS risers on my own car and I can't overstate how impressed I am with the kit and their customers service, plus the car looks awesome

 

Interested to hear how you get on, this has happened on my 2020 Club Edition! As Kaviar says, it does seem like it's metal rather than plastic.