As a follow-up to, let's remove the hook-based plugin discovery. It's not being used in core and between yaml and annotation-based discovery (with derivatives) it would seem like 2 ways is sufficient and more makes the DX worse.
Remove the \Drupal\Core\Plugin\Discovery\HookDiscovery class and any references to it in comments or documentation.
Remove \Drupal\Core\Plugin\Discovery\HookDiscovery so discovery should be more standardized on YAML or annotation discovery
PASSED: [[SimpleTest]]: [MySQL] 58,329 pass(es).
FAILED: [[SimpleTest]]: [MySQL] Failed to run tests: tests were executed, but no results were found.
PASSED: [[SimpleTest]]: [MySQL] 58,545 pass(es).