Coronoro

Simple Plugin which shows whether a registered user has read a Post or not. 
You can also choose to show the number of unread Posts at Forum level. 
This Plugin is an update to the older <a href="https://wordpress.org/plugins/bbpress-unread-posts/" rel="ugc">bbpress-unread-posts</a> Plugin.

bbPress Unread Posts v2

Simple Plugin which shows whether a registered user has read a Post or not.

i.lychkov

Highlight unread posts on your blog.
Key features:
<ul>
<li>Works right out of the box</li>
<li>Uses cookies, no authorization required</li>
<li>4 different types of markers for highlighting posts (a “new” text label, 
an orange circle, etc.)</li>
<li>Customizable background color for unread post titles</li>
</ul>
There’s a setting to determine when a post should be considered read. You can pick one of the following:
<ul>
<li>after it was opened</li>
<li>after it was displayed in the post list</li>
<li>after opening any page of the blog</li>
</ul>
Functions available for theme developers:
<ul>
<li>mnp_is_new_post($post) – check if a post is unread</li>
<li>mnp_new_posts_count($query) – get unread posts count</li>
</ul>

bbPress Unread Posts v2 vs Mark New Posts