Yeah it is a gearbox off job for the slave cylinder unfortunately.
Crack off the slave cylinder bleed nipple and push the pedal down to see if fluid comes out. If it does, we can outrule the Master cylinder.
If nothing comes out with the nipple cracked off, maybe that plastic donut thing near the ABS pump is busted. It's some kind of clutch delay valve has 'things' inside it which potentially could break.
And lastly, if they both pass, logically it has to be a jammed clutch, although I don't quite know how or why! It's a puzzler! Not heard of pedals not going down before, very odd.