Are shared user tables still supported and working?
Don't.

Per table-sharing is a relict of the past, deprecated and will be removed in Drupal 9. With all the layers on top that dynamically create tables for fields, cache invalidation and so on, this is never going to work.

If you want to have a shared authentication system, then look into an actual single sign on projects.
These are some links to get started; I don't have a lot of experience with either yet.


https://www.phase2technology.com/blog/implementing-cas-single-sign-on-in-drupal-8/
https://dev.acquia.com/blog/drupal-8-module-of-the-week/drupal-8-module-of-the-week-simplesamlphp-authentication/05/04/2016/10246