After 2 years, a fresh D9-enabled version, including the following fixes:
Issue #3078044 by stefan.butura, johnv: Check if Transition is executed twice for non-default revisions
Issue #3096439 by mhmd: Add redirect/cancel Url for non-node entity in RevertForm
Issue #3022421: Drupal Coding Standards
Issue #3078046 by stefan.butura, leontin: Incorrect previous StateID after changing older revision
Issue #3046122 by donaldp: WSOD with Fatal error: Call to a member function cachePerPermissions() on boolean in src/WorkflowAccessControlHandler.php
New: Plugin settings moved to "Content authoring" sub-category.
New: Rework all library classes.
New: Whitelists support for SFW.
Fix: Issues with large SFW databases.
Fix: Alternative cookies issues.
Fix: Javascript checking issues.
Fix: Multistep forms issues.
Other minor improvements.