I tried the Drupal 8 version of the Mass Contact module (8.x-1.0-beta2, released on June 2017).
As I was not fully satisfied, I decided to uninstall it.
Once done, I got errors when I tried editing a user account.
The website encountered an unexpected error. Please try again later.
Error: Call to undefined function
options_allowed_values()(line 89 of core/modules/options/options.module).
Digging a little bit, I found that the code to uninstall the module was not fully implemented: it leaves (at least) two configurations (field.field.user.user.mass_contact_opt_out and field.storage.user.mass_contact_opt_out) and a table in the database (d8_user__mass_contact_opt_out).
How can I get rid of that and have a clean uninstall?