In the previous versions, you could simply create the necessary directories (site.example.com/settings.php and files/ for instance) in order to get Drupal to look for the proper config and files for the correct multisite currently requested in the browser.
In Drupal 8.2.6, there's a "required" sites.php file that must exist in order for any of this to work. In my opinion (I'm sure someone somewhere has the same opinion), this file should NOT be required... but it is. Let's look at an excerpt:
* This file is required for multi-site support and also allows you to define a
* set of aliases that map hostnames, ports, and pathnames to configuration
* directories in the sites directory. These aliases are loaded prior to
* scanning for directories, and they are exempt from the normal discovery
* rules. See default.settings.php to view how Drupal discovers the
* configuration directory when no alias is found.
that last sentence: "See default.settings.php to view how Drupal discovers the configuration directory when no alias is found." is key. Why can't the configuration directory be discovered in the same way if this file doesn't exist? I understand that if you have multiple sites that you want to use the same configuration directory (the point of this file is to set up aliases specifically for this purpose) then you want the file to be properly set up to help Drupal find the correct setup for the current site.
However, in my recent (now) experience, I only need to set up multisite to have a few sites use the same application. All different domains. All different databases.. that's it. My fix was to simply create the file. No need to add any code to it. With the file essentially empty, Drupal has defected back to the way it found site configs in D7: name a directory sites/example.com, throw a settings file in there with database credentials, and start your install.
Just create that sites/sites.php file and D8 will be happy with your D7 multisite config methods