unicode_requirements() is deprecated

In previous major versions of Drupal, unicode_requirements() was a helper function in an include file that was intended to improve code readability in the long system_requirements() hook implementation. However, procedural include files and global functions are no longer a best practice.

Since the function was intended as an internal helper function and is not a hook_requirements() implementation in itself (despite its name), the logic is moved inline into system_requirements(). unicode_requirements() should not be used.