After MDL-30592 we have two functions 99.9% equivalent:

• plugin_callback() the "old" one, with separate $type, $name, $feature, $action, calling always to
• component_callbackup(), with just component and function

This issue is about to decide if we want to deprecate the former, moving all uses to the later. Or, alternatively, if both can coexist forever.

(deprecation would happen in current master - 2.3 -, and drop support in next one - 2.4)