While dealing with #1844 the question from the subject came up.

Our position in this is best described in the #1844 (comment) by @wjkohnen:

We still can reproduce stale environment variables as mentioned in 1. of #1844 (comment). When we manually add, change, delete (kubectl edit deployment) environment variables, we had expected a chart upgrade to revert any of these changes so as to reflect the state that is expressed in the chart. Any deviance from the expressed state of a chart in our view degrades the idempotency of helm releases and thus the idea of k8s clusters to be "self-healing". After spending some time tackling these issues I have some intuition that this desired behavior is (currently) not feasible. We'd appreciate some clarification.