The reason why collapsing was disabled in Camunda Modeler is that it’s right now not well supported in Camunda Platform. For example, in Cockpit you can expect dangling links to the children elements.
More sources on collapsed subprocess and how to replace it: Remove the ability to model collapsed sub-processes · Issue #1486 · camunda/camunda-modeler · GitHub, How to link a collapsed subprocess to a model, Collapsed subprocess not executable - #2 by Niall
Camunda Modeler 3.5 Released - Camunda