A slew of improvements to CDCR viz.
SOLR-6465, SOLR-8389, SOLR-8391 are planned and on which I am actively working. Unfortunately, those won't make it to the 6.0 release. At the same time since both the amount of configuration and mode of configuration (API vs editing solrconfig.xml) will change with these planned improvements, it'd be nice to not have strong back-compat guarantees which prevent us from making CDCR easier to use.
Therefore, I propose to put a note in CHANGES.txt and any documentation in the Solr reference guide to this effect as well as add an experimental response key in all CDCR API responses.