An updated version of WordPress introduced a deprecation warning that appeared at the top of pages.
This problem affects a handful of sites that had advanced logging enabled. It was unlikely to affect production feeds - more likely to be staging or UAT feeds that have extra logging. As production feeds were not affected, we have downgraded the impact to Major from Critical.
For reference, the warning was.
<b>Deprecated</b>: class-wp-feed-cache.php is <strong>deprecated</strong> since version 5.6.0 with no alternative available.
This is the root issue:
https://github.com/radgeek/feedwordpress/issues/127#issuecomment-762513715