Are all updates (revisions) to a user recorded in the logs? A user was created just over a week ago and at some point was given a particular role by an administrator. Will this update (the role given, the date/time of the update and the admin who made it) have been recorded by default in the database somewhere? I don't have any special user modules installed on the site e.g. User Revisions. When I tested just now by giving roles to a user and then looking in "recent log messages" (filtered to "user"), my update didn't show there, so I'm guessing the answer's no, but if there is a way to retrieve this info it would be helpful.
Unfortunately not: Changes to a user are not logged (even though newly created users are), and the user entity does not support revisions in core (as of Drupal 8.4.2).

You could add an extra submission function with hook_form_user_form_alter() that creates a log message.