Hi Dave,
That Drupal project is not maintained or supported directly by Constant Contact, so I can't really provide much feedback on if this is functioning correctly. What I could suggest as a workaround is to unsubscribe the customer and then add them back in to the correct lists, it will take two steps to do the one goal but you will not have to write any additional code to use the REST API.
Regarding the legacy APIs, information regarding their future will be made to developers when we have more to provide. Our current plans are as stated, to withdraw support for developers in the future with no exact date decided yet.
Dave Berard
Senior Product Manager, Constant Contact