If it's just 1 or 2, rest 180 or 200 grit sandpaper against them and scribble on the back with a pencil. It will take away the shiny and give a matte finish, and it's good enough for photos.

If it's lots, you'll need a metal baking tray, spray vegetable oil, some fine soft sand, a tub of warm soapy water, the posts, some locking pliers/grips, and a chicken.

Sort the keys in order from best to worst. Put the posts in the back of them so you can pick them up. If any posts are loose, resolve that first.
Lay the sand in the tray, level it smooth but don't compress it. Next, spray with canola oil. Heat in oven until it just about starts smoking.

The glass transition temp of ABS is 105C. The smoke point of Canola is 107C.

Going from best to worst, lift the keyscaps up, and lower them into the sand, level, and depress them in until the keycap is level with the sand surface. Count a couple of seconds then pull it out and put it straight in the water. The stuck soft grains of sand will wash right off (unless your sand wasn't properly oiled) and you can check the finish of the key. If you're happy with it, do the next one. And the next one. Once you get into the shinier keycaps you might need to do 3- or 4- seconds. Remember, the sand is also cooling so you'll have to heat it up again to >110F or so. The worst worn keys are always the same: space, E, O, I, U, S, A, N, :, enter.

Once washed and dried the keycaps will not only be non-shiny, but they'll have a common and matching texture that's very similar to new keys.