On the engine level, Flowable does support conditional event. However, not as a start event of a regular process. It is supported for an Event Sub Process Start Event, Intermediate Catch Event and Boundary Event.
Are you sure that other engines support it on the top process level, and not an Event Sub Process?
In any case, Flowable has CMMN, which together with BPMN might be a better fit for what you need.
the camunda documentation has two separat sections for ConditionlStartEvents and CondionalStartEvents for sub processes. Though we thought that’s the case.
I will have a look at the information you provded.