A couple of points:
1. I was a little unclear on when you wanted to send the email. If you want to send an email when the Change Management plan is written/developed - the closest you can come is using the "OnPlanningCompleted" ruleset, which will run when the Change Manager completes the plan approval step.
The "OnImplementationCompleted" ruleset will run when the final Change implementation is actually completed and the Change is closed. (This may have been your intention).
2. The "${ImplementationPlanCompletedOn}" field is only populated when the Implementation Plan is created via a "Delegate Plan" task at the "Change Manager Review and Approval" phase. (See screenshot). If the plan is completed during initial Change request or via the "Edit Change Plan" Process Action, the Implementation Plan is considered to have never been "Completed" and the field will not be populated.
You can see this field's contents on the Change Process View Page, under the Implementation Plan pane. (See screenshot)
Some of the available fields for the Email Templates are a little tricky like this - not all of them are valid for different paths through the process, and some get populated (or cleared) only under some conditions. Additionally, if you try to use a field that could not have been populated yet in the process flow - you won't get any data. (For example, using the "CloseCode" field at the "OnChangeReceived" ruleset obviously doesn't make sense).
Hope that helps - good luck!