FILTER_SANITIZE_STRING is deprecated as of PHP 8.1
As of PHP 8.1, FILTER_SANITIZE_STRING is deprecated. Source: https://www.php.net/manual/en/filter.filters.sanitize.php
Deprecated as of PHP 8.1.0, use htmlspecialchars() instead.
We use FILTER_SANITIZE_STRING here:
https://github.com/wpengine/atlas-content-modeler/blob/53331a7d1ee34bcf739e2226fbc2cd934130e5f8/includes/settings/settings-callbacks.php#L81-L82
FILTER_SANITIZE_FULL_SPECIAL_CHARS looks like a good replacement.
See Atlas Search change here: https://github.com/wpengine/wp-content-engine/pull/552/files
Jira: https://wpengine.atlassian.net/browse/MTKA-1465
Fixed in https://github.com/wpengine/atlas-content-modeler/commit/d8461cfebe37352a6fb9e97b9340af4b50d0f4dd