The is a "lift shaft" type area located between the platforms towards
the middle. It's now a pump room (the pump was prone to fail not so
long ago, causing flooding on the platform). As others have said, the
escalator shaft towards Broad Street was located at the west end of
the platforms, up some stairs and along a passage, coming out where
the bus station now is.
So I assume its been capped now at the ticket hall level, so that they
can have that newer set of escalators to the north?
I think it's to the more extreme West of the site than that.
I had a little wander around Liverpool St the other day, and at the far
western end of the eastbound platform is a footbridge over the line, which
would make sense with an exit at that end going up a few steps then turning
due north and crossing the platform.
I've noticed that, the current access to it is from the door directly
next to it on the platform - it goes up about 3 steps towards the
escalators then its a wall and you have to turn round to go up to the
bridge. I'm not sure if that's a new access to an old bridge. To me,
it seems to be too far north from liverpool street (the street) to go
directly to some sort of access to (the former) broad street station -
its closer to being directly underneath that modern shopping arcade
between liverpool street station and broadgate.

There's little room for anything else to be using that bridge, as the
westernmost of the three banks of escalators has already cleared that height
at the point where the line is crossed.
But only the middle bank of escalators is original. And the
westernmost one was most likely only added when they rebuilt liverpool
street station in the 1990s