TYPO3 Core

The condition should be removed in ext_localconf.php.

By limiting the wizard PageTS to Backend only, the content element type will not be listed when in frontend context, and thus it will not be available in the Frontend Editor.

Suggested change:

Before

// Add login to new content element wizard
if (TYPO3_MODE === 'BE') {
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig('
        mod.wizards.newContentElement.wizardItems.forms {
            elements.login {
                iconIdentifier = content-elements-login
                title = LLL:EXT:backend/Resources/Private/Language/locallang_db_new_content_el.xlf:forms_login_title
                description = LLL:EXT:backend/Resources/Private/Language/locallang_db_new_content_el.xlf:forms_login_description
                tt_content_defValues {
                    CType = login
                }
            }
            show :=addToList(login)
        }
    ');
}

After

// Add login to new content element wizard
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig('
    mod.wizards.newContentElement.wizardItems.forms {
        elements.login {
            iconIdentifier = content-elements-login
            title = LLL:EXT:backend/Resources/Private/Language/locallang_db_new_content_el.xlf:forms_login_title
            description = LLL:EXT:backend/Resources/Private/Language/locallang_db_new_content_el.xlf:forms_login_description
            tt_content_defValues {
                CType = login
            }
        }
        show :=addToList(login)
    }
');

Related issue for frontend_editing: https://github.com/FriendsOfTYPO3/frontend_editing/issues/263