The check_markup() function is deprecated

The check_markup() function is deprecated and no replacement is provided. It's always recommended to return a renderable array, when possible, without flattening as markup, to preserve the cacheability metadata.

Before

$formatted_text = check_markup($text, 'basic_html', $langcode, $filter_types_to_skip);

After

$formatted_text = [
  '#type' => 'processed_text',
  '#text' => $text,
  '#format' => 'basic_html',
  '#filter_types_to_skip' => $filter_types_to_skip,
  '#langcode' => $langcode,
];

Particular cases:

• Mail: Future Drupal mail system development will pipe the mail preparation through Twig (for both, plain and HTML messages). This will remove the need to manually render the renderable array (see #3539651: Introduce email plugins). Until that API is available, 3rd-party code could render the above array build, but only cases such as email body, where the cache metadata makes no sense.
• Tests: Many tests are only asserting that a text has been formatted correctly. For convenience, Drupal is providing the \Drupal\Tests\filter\Traits\ProcessedTextTestTrait trait with a helper method, facilitating the testing.