What would you like to be added:

As preparation for PipeCD v1 release (scheduled in the end of this year), I want to clean up the codebase. As the result of this issue, features marked as deprecated would be:

• Deleted from the codebase (controlplane codebase)
• Has a clear decision that whether we bring that feature to the v1 codebase or not

This issue will be used to track the progress of deprecated features removal.

Note: The deprecated feature on pipedv0 could be remained in pipedv0 codebase, and will not be included in pipedv1.

Why is this needed: