This was detected as part of MDL-73587 (PHPCompatibility complete run against codebase).

The error says:

'mixed' return type is not present in PHP version 7.4 or earlier

And there is one case in:

• lib/phpspreadsheet/vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Collection/Memory/SimpleCache3.php : 54

We need to check if that can hit us or maybe it's unused code and can safely ignore it.