What was "Old Hatch" ?

It used to be based on a combination of remaining survivors and the number of generators left.

If all 4 survivors were alive, the hatch couldn’t spawn until the exits were powered. For each survivor that was no longer in the trial, the hatch would appear 1 generator earlier. So with 3 survivors it appeared when there was 1 gen left, and with 2 survivors it appeared at 2 gens left. However, it would always spawn closed if there was more than 1 survivor left, and could only be opened with a key. It could spawn as soon as a generator is completed (causing that threshold to be reached) or immediately following the death of a survivor, if that lowered the gen-threshold sufficiently to allow it to spawn).

With 1 survivor left it appeared no matter what (this is the only situation in which it appears today). Once it had spawned and 1 survivor was left then it would open on its own.

The hatch also used to only stay open for a certain amount of time when opened by a key, if there was still more than 1 other survivor left after the survivor who used the key escaped through the hatch. This was originally 30 seconds, but was later reduced to 10 (not long before the hatch was changed to what we have now). During the time it was still open, other survivors could escape through the hatch, so only 1 key was required for multiple survivors to escape and was the main reason keys used to be considered wildly overpowered. If this time elapsed, then the hatch would shut on its own without starting the EGC. If the killer closed it before that happened, it would auto-complete the remaining generators and start the EGC like it does now, even if there was more than 1 survivor left. Using a key was also instant (today it takes 2.5 seconds and you can be grabbed by the killer while doing it).

Prior to the EGC existing, killers didn’t have the ability to close the hatch manually but the action of jumping into it could be interrupted by the killer leading to standoffs.