empty gun
dbl check empty chamber
no mag
retract slide back to cut out over the slide stop pin to allow clearance for removal.
depress slide stop pin from back side.
if stubborn and you say might be years of never taken apart and might be built up residue which is causing the stuck effect, heavy penetrating oil and let sit for a while.
this is how I pull back with one hand and use the other hand to depress the slide stop pin for field strip
if stubborn, try taking a non marring short punch (with ID just a tad smaller than the slide stop pin) in a vise well protected with cloths wrapped around the vise to protect the gun finish.
only expose the punch at a 12 O 'Clock position about a 1/2" sticking out of the vise.
retract the slide in the position shown above and hold and somehow place the side of the slide stop pin on the exposed punch and push the gun down to depress the slide stop pin.
you may want to also put a couple of layers of masking tape on that side of the gun to avoid scuffing or scratching.
just a suggestion
I am sure a non marring mallet and holding the slide back to the position described above and a tap or two would get it to move also.
good luck