Jeff Starr

Override WordPress “automatically close comments” option and leave comments open on any individual posts.
Want to keep comments open on a few old posts? This plugin is for you. Here’s how it works:
In the WordPress Discussion Settings, there is an option called “Automatically close comments on posts older than” whatever specified number of days. When this setting is enabled, comments will be disabled on any posts that are past the deadline, whatever “x” number of days.
What WordPress is missing is a simple way to override that global option on a per-post basis. That’s exactly what this plugin does. For any post that is past the comment deadline, this plugin will display a small meta box on the “Edit Post” screen for each post. There you can check the box to enable comments on the post. Comments will then remain open for that post indefinitely, until the box is unchecked or the plugin is uninstalled.
Note: The override meta box will not be displayed if the post is not past the comment deadline (whatever “x” number of days).
Check out the screenshot below to get a better idea.
Features
<ul>
<li>Clean, secure code</li>
<li>Written according to the WP API</li>
<li>Simple, lightweight, and flexible</li>
<li>Localized and translation ready</li>
<li>Cleans up after itself when uninstalled</li>
</ul>
For more details about how it works, visit the <a href="https://wordpress.org/plugins/override-comment-options/#installation" rel="ugc">Installation docs</a>.
Privacy
This plugin does not collect or store any user data. It does not set any cookies, and it does not connect to any third-party locations. Thus, this plugin does not affect user privacy in any way.
Override Comment Options is developed and maintained by <a href="https://x.com/perishable" rel="nofollow ugc">Jeff Starr</a>, 15-year <a href="https://plugin-planet.com/" rel="nofollow ugc">WordPress developer</a> and <a href="https://books.perishablepress.com/" rel="nofollow ugc">book author</a>.

Override Comment Options

Enables per-post override of "Automatically close comments on posts older than" option in the WP Discussion Settings.

Huda Toriq

CommentMailer is a WordPress plugin that generates and sends e-mail to your blog’s commenters to tell them that you 
have replied his/her comment on your blog’s comment system. It adds extra options on your blog’s comment form to select 
which commenters you are replying to. Submitting the reply comment will trigger the e-mail generation function thus 
send them automatically. You can customize your own notification e-mail or use the default one.

CommentMailer

Send automated email to your blog's commenters everytime you reply their comments on the blog's comment system.

Override Comment Options vs CommentMailer