A warning is given when trying to log into the backend. Additionally if $GLOBALS['TYPO3_CONF_VARS']['SYS']['exceptionalErrors'] is configured to turn warnings into exceptions no login into the backend is possible anymore.
(1/1) #1476107295 TYPO3\CMS\Core\Error\Exception
PHP Warning: Undefined array key "login-provider" in .../vendor/mediadreams/md_saml/Classes/Authentication/SamlAuthService.php line 118
Tested with md_saml master on TYPO3 12, PHP 8.1
A warning is given when trying to log into the backend. Additionally if $GLOBALS['TYPO3_CONF_VARS']['SYS']['exceptionalErrors'] is configured to turn warnings into exceptions no login into the backend is possible anymore.