Toilet shutoff type identification
It is a sweated valve with a screw on attachment for the toilet connection.
Rather than try to remove it, I'd buy a second one; they are typically quite cheap. With a helper available, use a giant wrench to try and break the valve free.  If the valve breaks, turn off the water; if it moves, great.
If it doesn't break free, take the handle off and then take the packing / valve mechanism out by unscrewing the front hex net and removing the valve innards.   Replace them with the parts from the donor valve.
If you get lucky, you'll save the time necessary to desolder, clean, and re-solder a valve.