I have seen some questions regarding BPMN, but not as a main topic. Are BPM and BPMN questions suited for this site? Would it be a better to create a SE site just for that?
BPMN Isn't Normally a Project Management Practice
Unless you're working on a project that has business process re-engineering as a deliverable, I'm not sure that the Business Process Model and Notation is directly relevant to the practice or profession of project management. There's certain to be some overlap, and project managers do sometimes wrestle with business process, but business process modeling is probably off-topic except insofar as it might relate to chartering or managing a specific project.
If you have an unusual use case that is on-topic for project management, that's a different story. However, questions about BPMN in the abstract, or about BPMN outside of the scope of project management, will definitely be off-topic and likely to be closed by the community.