Magento 2 Check Email id exist or not at website level.

Magento 2 handles the email address at the Global and Website level.

You can set global or website level for email id from Stores -> Configuration -> Customer -> Customer Configuration -> Account Sharing Options

Choose Share Customer Accounts Global or Website level.

How to delete CMS Page by Id programmatically in Magento 2?

You can delete CMS Page by Page id in Magento 2 using PageRepositoryInterface interface.

Magento\Cms\Api\PageRepositoryInterface interface is responsible for delete a CMS page from admin panel.

You just need to inject interface in your constructor to delete CMS page.

List of Reserved Keywords in Magento 2.

Magento 2 has a lot of reserved keywords and they define all the reserved keywords in their native core file.

You can find out all the keywords at vendor/magento/framework/App/Router/ActionList.php file.

Reserved keywords are not used to define any class name, function name or variable name in your code.