Seth Alling

Up until recently, WordPress gave two options: You could either disable comments and trackbacks by default for all pages and posts, or you could have them active by default. In WordPress version 4.3, this finally changed so comments are always disabled on new pages.
While the new change makes it easier for many of the sites, it make it harder for people who need to get the reverse and enable comments on all pages, or if they need to change the default for a custom post type. This plugin allows you to choose whether comments are enabled or disabled by default on all new posts, pages and custom post types, while still giving the ability to individually enable comments on posts or pages.
Also, this plugin provides a way to quickly disable all comments or pingbacks for a specific custom post type. It directly interacts with your database to modify the status, so it is highly recommended that you backup your database first. There shouldn’t be any issues using this feature, but it’s always good to play it safe.
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No Page Comment

An admin interface to control the default comment and trackback settings on new posts, pages and custom post types.

Robin W

Pascal Casier

Adds an option to the Discussion meta box to use a bbPress topic instead of WordPress comments, and displays that topic beneath the post on your site.
You can let the plugin create a new topic for you in the forum of your choice, or specify an existing topic to attach to the post. 
A topic can be attached to as many posts as you’d like, but only one topic can currently be attached to a post.

Post Comments as bbPress Topics

Great support!

I use it to link my Learndash to bbpress forums so that my students can discuss a lesson, either under it or in a forum topic. Really practical, glad that this plugin exists!

Really practical, glad that this plugin exists!

Replace the comments on your WordPress blog posts with topics from an integrated bbPress install

No Page Comment vs Post Comments as bbPress Topics