I was trying to diagnose if my o2 sensor is dead so I disconnected it and there was no change to how it idles, or performs under WOT, or at cruise. (at 100 with foot off the pedal the trip computer always shows about 2l/100km)

As I understand, the O2 sensor is only used in closed loop (idle and cruise).
I have a large cam fitted (CMS stg3) and trying to sort out an idle flare and fall as well as a big flat spot just off idle/cruise sometimes.
Disconnecting the O2 sensor has gotten rid of the flat spot but the flaring idle still occurs. Most likely the idle is a tune thing. I had a J3 tuned for the car but afterwards discovered that the TPS was set wrong and the cam wasnt dialled properly. Once I made these changes it pinged a lot so I removed the chip and havent got back for a new tune yet. This time though I want to make sure everything is working properly before I take it back.

This cam has very low vac at idle. Would this low vac be causing the ecu to stay in open loop, which would explain why there was no change when I disconnected it?

What other sensors cause the ECU to enter open loop mode?

the low manifold vac would be one of the main issues,
when the map sensor sees low vacuum the ecu will think its at partial load, combined with the tps voltage it will determin if the car is idling or not..
so if the tps voltage is to high and the manifold vac is low the ecu wont use closed loop for idle..
but to operate correctly in closed loop mode, the tps needs to be within the spec voltage .7v, and the 02 sensor needs to be operational. once the ecu gets the right tps voltage and can read o2 it will learn its closed loop and the ecu will learn around the low idle vacuum and make changes in the closed loop to try keep the afr at a reasonable level for smoother idle.

also an old non working 02 would more than likely either show a lean mix to the ecu or would not send any signal to the ecu at all..that could be the reason why un pluging it dont make a difference..

How low is your vacuum at idle in neutral ?