How can I remove orphaned Cohesion Layout Revision entities?

Use the tools included in the "Entity Reference Revisions" contributed module.

Drupal Core has an unresolved issue where entity reference field items are not being removed when the referenced entity is deleted. This applies to Cohesion Layout Revision entities. We recommend the "Entity Reference Revisions" contributed module (dependency of Site Studio) feature to fix this.

In version 1.7 of "Entity Reference Revisions" contributed module a new feature "Orphaned entity cleanup" was introduced and in version 1.9 a new feature "Implement a Drush command to clean obsolete composite entities" was introduced. These two features apply to composite entities such as Cohesion Layout and can be used to clean up orphaned revisions.
This can be done via UI by navigating to https://<your.hostname>/admin/config/system/delete-orphans or by using a Drush command "drush err:purge cohesion_layout".

For more information, see Managing revisions.