The submarine had to (survive) a combat before submerging.
In Classic 3rd edition (defending only, no subs can submerge in Classic 1st and 2nd editions) and Revised, yes. In original Europe and Pacific, attacking subs can submerge immediately, while defending subs must survive attacking fire first.
Sub Stalls are more of a relic from classic than any of the other versions.
I think you mean Revised, as subs had no effect on transports’ loading in Classic. Also, because attacking subs can submerge before any dice are rolled in original Europe and Pacific, substalling is much more effective there, especially in Pacific.