It is seized because the piston is rusted to the bore.
It is salvageable & useable, but to be useable it will need to be sleeved by a company like White Post. Sleeving it bores out the inside a bit more & inserts a non-corroding brass sleeve for the piston to travel in.
It should be salvaged if possible, because the next rebuild/repairs will be less expensive, & replacement parts more & more do not look the same, perform as well, &/or last as long.
My advice would be to get a new MC now to put on the car, & start work on dismantling the seized one. Then store the disassembled original so that when the replacement starts to leak & needs a rebuild, you can send the disassembled original off for re-sleeving & just swap it onto the car. Future rebuilds will just require replacement of the seals.
The piston is not likely to have corroded to the point of unuseability. White Post's instructions (from back when they just sleeved the cylinder & didn't rebuild the whole thing) mentioned that if the piston has jagged pits, etc. to just dress them up (smooth the jaggies down) with some fine emery cloth. This is because the piston in operation doesn't contact the cylinder walls; the replaceable rubber seals on the piston are what make contact.
To get a stuck piston out:
Even if it's a good condition MC, the piston is often difficult to remove. Forcing it out with compressed air or forcing in grease via a grease gun doesn't work well with a dual MC because of all the other holes. The method is to use inertia: get a 2x4, drill a hole in it larger than the big hole on the front, put it on the floor or bench (or carefully arrange 2 2x4's slightly further apart than the big hole).
Grasp the MC very firmly with some large, long-handled Channellock-style pliers
Then with a long, easy, tennis-style swing, whop it down on the 2x4 arrangement so that the wood stops the flange on either side, but nothing is in front of the hole in front. Inertia will cause the piston to keep traveling forward & out. You may need to do it several times, but it should be the long, even, firm swing. Whapping it with short swings doesn't do much.
As corroded as yours is, you'll likely need to soak it for a long time in penetrating oil, just throw it in a container with Marvel Mystery Oil or something for a week or more. If it still won't come out, you'll need to wash out all the oil (and dry it to inhibit more rust), then heat the MC as hot as possible immediately before swinging & whopping.
It's a procedure you'll need to master & is annoying on the learning curve, but brake hydraulic rebuilds on these cars are sort of regular occurrences. Fortunately they're really easy on cars that have been regularly running, as opposed to your long-sitting up example.
Dismantle the MC, store it (with a film of oil on it to inhibit rust), when the next rebuild time comes, send it off to be re-sleeved. This is one of those OEM parts which hasn't been improved in later available parts. I wish I'd saved all the master & slave cylinders I tossed in the trash during the years of easily-available original BL/Stanpart/Lockheed/Girling parts.