Seems like Sylvanas knows a lot about the Maw, Jailer and Shadowlands even before the events after BfA.
If Sylvanas died in the hands of Arthras, then she spent moments in the after realm for the dead for some quite time and be much familiar with the place.
Also when she commited suicide, perhaps it was her own way to make contact with the realm since she is "a cat with nine lives" and cannot visit there physically yet.
Does it mean she is already allies with the Jailer way back WC3:TFT since she's pretty cool and chill with Kel'thuzad and the Nathrezims. And she has an idea of whatever plan the jailer might be at.
Did her power to break free from the bonds of Arthras weaking control probably came from Shadowlands itself and the concept for freewill among the dead in line with breaking free from being a prison of the death-life system and has connection with the Jailer being chained and being jailed?