Problem/Motivation
Plugin implementation is specifically called out as not being BC. Migrate Drupal's yml backed plugin implementations therefore do not need to be BC. Let's make that distinction very explicit and documented.
Proposed resolution
https://www.drupal.org/core/d8-bc-policy#plugins
Discusses that plugin ids cannot change but only only calls out plugins backed by classes. Let's make that more specific.
Suggested improvements:
Particular plugin classes or yaml backed plugins should not be considered part of the public API.
https://www.drupal.org/core/deprecation#how-plugin Only addresses classes. It does not address yaml backed plugins without a UI.
Suggested improvements:
For yaml backed plugins, i.e. migrations, use hook_migration_plugins_alter and add
@trigger_error('...', E_USER_DEPRECATED)
Remaining tasks
Agree among ourselves