The ultimate goal of this issue is to remove \Drupal\Core\Messenger\LegacyMessenger entirely (which is deprecated and scheduled for removal prior to 9.0.0) and solely rely on the Messenger service.
If possible, 3rd party code should use the Messenger service or find alternative solutions that don't involve using this deprecated class.
If there is any 3rd party code that provides a legitimate use case for keeping this around, we may have to re-think how messages work (highly unlikely, but currently a complete unknown).
This is partially due to the fact that the drupal_set_message() and drupal_get_messages() proceedural functions have been around for over 14 years and usage of these functions was heavy throughout core, contrib and 3rd party code alike.
Thus, this issue also serves as a way to track usage of this deprecated class.
Use cases:
- Drush - Used when creating a new site and the container is not yet initialized. Unclear if this is a hard dependency or its code can be fixed, needs research