Hi Debraj, By Source Legacy Thread we refer to all sources which do not implement the new interface yet [1]. Currently only the Hive, Kafka and FileSource are already migrated. In general, there is no sever downside of using the older source but in the future we plan only to implement ones based on the new operator model.
Best, Fabian [1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-27%3A+Refactor+Source+Interface
