Posted by jefflogan on May 14, 2018 at 10:02pm
I have created a new module for Drupal 8, which provides a new condition for HTTP 40x client error status code pages.
This allows for useful things like allowing for blocks to be positioned on 401, 403 and 404 status error pages.
Let me know of any questions/feedback regarding the module.
I don't believe there are any similar modules yet, but let me know if there are any which potentially overlap.
Project page: HTTP client error status