Use WP Customize to modify elements of password protected and private posts and pages.

Make the WordPress post state text (draft, pending, sticky, etc) stand out.

Convert Protection Text は WordPress で記事（ページ）の公開状態を限定公開（「パスワード保護」）にしたときのテキストをカスタマイズするプラグインです。

Reset post or page password automatically after X days.

Share password protected posts via secret URLs

The Whitelist plugin enables you to open your website to a set of users and blocks everyone else. You can enter users by their email address into the Whitelist tab of the plugin. Once done, only users on the list would be able to visit your site.