In https://www.elastic.co/guide/en/apm/server/master/kafka-output.html and https://www.elastic.co/guide/en/apm/server/master/logstash-output.html (for every release version) it says that the default value for enabled is true, which it is ambiguous at best.

AFAIK those enabled are true "by default" only if (the whole output.elasticsearch sections is commented out or output.elasticsearch.enabled is false) and the corresponding section is uncommented (output.kafka or output.logstash).
Since by default that is not the case, you can't really say that output.kafka.enabled and output.logstash.enabled are on by default.

IMO we should be explicit and document it as I said above, or event better just mention that kafka / logstash enabled settings are disabled by default and add enabled: true in the example snippets.