Problem/Motivation
\Drupal\migrate\Plugin\Migration::get() has been deprecated since Drupal 8.1. It is an artifact of the heady days when migrations were config entities, and was kept as a BC layer when they were converted to plain plugins.
Proposed resolution
Remove this method in Drupal 9.
Remaining tasks
Do it.
User interface changes
None.
API changes
Deprecated method removal.
Data model changes
None.
Release notes snippet
None?