bbPress.org

The closest thing I know of is “Comment Quicktags”:

https://bbpress.org/plugins/topic/quicktags-4-bbpress/

I think a WYSIWYG editor would be overkill since a lot of the markup is not allowed by default. I suppose you could disable the buttons that use non-valid markup.

Or, these forum posts:

https://bbpress.org/forums/topic/i-added-wysiwyg-to-my-bbpress

https://bbpress.org/forums/topic/bbpress-wysiwyg-status