TYPO3 Core

\TYPO3\CMS\Core\Resource\Folder->getFiles() has several parameters, the third one is $filterMode:

    public function getFiles($start = 0, $numberOfItems = 0, $filterMode = self::FILTER_MODE_USE_OWN_AND_STORAGE_FILTERS, $recursive = false, $sort = '', $sortRev = false)
    {
        // Fallback for compatibility with the old method signature variable $useFilters that was used instead of $filterMode
        if ($filterMode === false) {
            $useFilters = false;
            $backedUpFilters = [];
        } else {
            [$backedUpFilters, $useFilters] = $this->prepareFiltersInStorage($filterMode);
        }

In TYPO3 6.0 in change 15405: [BUGFIX] Folder: File list filtering does not work the signature of the methoed was changed from
public function getFiles($start = 0, $numberOfItems = 0, $useFilters = TRUE) {
to
public function getFiles($start = 0, $numberOfItems = 0, $filterMode = self::FILTER_MODE_USE_OWN_AND_STORAGE_FILTERS) {

It introduced the filter mode constants by renaming the third argument from $useFilters = TRUE to $filterMode = self::FILTER_MODE_USE_OWN_AND_STORAGE_FILTERS

The logic is meant to do the following: In case of self::FILTER_MODE_USE_OWN_FILTERS and self::FILTER_MODE_USE_OWN_AND_STORAGE_FILTERS the filtering in the folder for this Folder->getFiles() call is not only done by the filters existing in the storage, but additionally those in the Folder. For the actual filtering to happen in the storage the storage->fileAndFolderNameFilters must be changed. In order to be able to restore the former state the previous filters are saved in $backedUpFilters to restore them after the operation.

The change 23398: [BUGFIX] File identifiers are case-insensitive included in TYPO3 6.2 set $backedUpFilters = []; if $filterMode === false. This was a massive change in another scope, and it looks like the author just wanted $backedUpFilters to be set as well in the if as the else case and initialized it in the wrong way.

$filterMode = self::FILTER_MODE_NO_FILTERS will set $backedUpFilters = null;
$filterMode = false will set $backedUpFilters = [];

As restoreBackedUpFiltersInStorage() only continues if $backedUpFilters !== null there is a difference between these seemingly equal filtermodes: $filterMode = false will "reset" the $storage->fileAndFolderNameFilters to an empty array while $filterMode = self::FILTER_MODE_NO_FILTERS will not change $storage->fileAndFolderNameFilters at all.

This means $filterMode = false will reset the $storage->fileAndFolderNameFilters to an empty array permanently even if the storage contained filters before. I guess this is a bug. As 23398 was part of TYPO3 6.2 the core did not use the fallback itself back then and this bug went by unnoticed.

Long story short: This fallback is very outdated and even buggy, it should definitely be removed.

Whether breaking immediately or with deprecation I can't decide, but I vote remove it