I don’t believe so.  In the same way that unpublishing a course doesn’t impact the completion data or previously awarded certificates for an individual course, unpublishing a Learning Plan should follow the same logic and retain the completions and certificates.  I believe deleting is the only action that would potentially impact the certificate, in this case.

At first glance it also appears that even when the Learning Plan itself is under maintenance, users can still access the individual courses that are assigned to them through those learning plans unless every example I checked was in a cross-posted LP.