Does the newly introduced cef processor require license?
It comes with the basic (free) license.
I upgraded my staging node to 9.3.4 and can't find it in ingest pipelines or by using dev tools.
It is likely that Kibana just hasn't been updated to know about it yet. The processor is present in the free tier in 9.3.4. If you are building your pipeline using the elasticsearch API, you ought to be able to add a CEF processor by giving it type "cef".
A colleague pointed out to me that this functionality is coming to kibana soon: [Ingest pipelines] Add CEF processor by SoniaSanzV · Pull Request #266352 · elastic/kibana · GitHub.