I raised a ticket with Matlab support, there is a solutiong that can load Model Linearizer on the online version as below:
1. Launch MATLAB Online.
2. Run this command in the Command Window:
learning.simulink.launchOnramp('controls')
3. Close the Simulink Onramp window that should open shortly and navigate to your model in Simulink Online.
4. Go to Apps and inspect the "Control Systems" section.