As per this issue I see that we earlier mentioned that users can use drag/drop to move issues out of an active sprint.

Click Plan, then drag the issue out of the planned sprint and into another sprint or into the backlog.

However, this was not working so we fixed it in the documentation.
As a user this would be the most intuitive way to do it. Currently there are no hints as to why a user is not able to drag drop an issue out of the sprint in the Plan mode. We should fix that.

Regards,
Ruchi