Are shared user tables still supported and working? I have tested a Drupal 7 multi-site before, and it worked for me perfectly. With Drupal 8, it doesn't work as expected.

Suppose there are two sites, site1  with site1_ as table prefix and site2 with site2_ as table prefix.
The documentation says that I should add the following lines in the settings.php file.

  'prefix' =&gt; array(
     'default'   =&gt; 'site2_',
     'users'     =&gt; 'shared_',
   ),


It doesn't work because the users table in Drupal 8 doesn't have username and password information.

If I add this lines, Drupal won't work normally too.

  'prefix' =&gt; array(
     'default'   =&gt; 'site2_',
     'users'     =&gt; 'site1_',
     'users_data'     =&gt; 'site1_',
     'users_field_data'     =&gt; 'site1_',
     'user__user_picture'     =&gt; 'site1_',
   ),


For example if a user in site1 changes password, this user cannot login with the new password on site2, but the user can login with the old password once, and sign out; then the new password works!

I expected that the migration path from Drupal 6 to Drupal 8 would support migration of the shared table too, but this doesn't seem to be true. Even reading https://www.drupal.org/node/2551549 and https://www.drupal.org/node/2768219 doesn't help me much. 

Are shared user tables still supported and working, on Drupal 8?
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