[2.7] adds deprecation notices.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

@hhamon no. We need to be able to instantiate this class without any warning, because it is used by our BC layer.

What we need is having deprecation messages in the BC execution paths only (the places where the parent logic is not executed directly)