Hi! tell me how to remove the www domain, on hosting in the htaccess file  

RewriteEngine On

RewriteCond %{HTTP_HOST} ^www.site.ru$ [NC]

RewriteRule ^(.*)$ http://site.ru/$1 [R=301,L]

The hosting's response is technical support:

Most likely, the problem when adding redirection rules to. htaccess is related to errors caused by internal routing of scripts.
Unfortunately, we do not know the structure of the scripts of the site used.
In this case, I recommend that you additionally contact the developer to add the correct redirection rules from the www subdomain to the main domain

Inside of your conf_global.php will be: 

'base_url' => 'https://www.site.ru/',

Change this to:

'base_url' => 'https://site.ru/',

Basically just remove the www reference.  🙂

Thank you, all good)) 

Once you've done that, make sure to go to the ACP -> Support page. Once the page finishes loading fully, click the button on the right to clear the system cache.

Then just navigate away from that page. Those actions should clear any instances of www in the system links, except for those posted directly in a topic (since the data is saved "as-entered" and is not dynamic).

You're most welcome. If you have any issues, please feel free to update this topic, but I think you're good to go.