When you send a request from an external API to a Drupal website without this header then you see the following error:
Deprecated function: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in Mobile_Detect->match() (line 1460 of /var/www/vendor/mobiledetect/mobiledetectlib/Mobile_Detect.php)
It was solved here so we need to update a required library to the next version.
Fixed
3.0
Code