sounds like most users just want bbpress forums to use “no sidebar” page template, and they have a page called “Forum” but page-forum.php does not work and you can’t also change the page template inside the post editor window either, this is because bbPress does not consider that to be a page… it is archives.

so you can also use archive-forum.php instead of forum.php

bbpress documentation is really bad and outdated btw….

Pretty much you would need to have a template like forum.php and then instead of having a normal loop like:

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
    <?php the_content(); ?>
<?php endwhile; endif; ?>

you have to call the loop of the specific page you want to use, eg. /forum/ or whatever.

$id=47;
$post = get_page($id);
$content = apply_filters('the_content', $post->post_content);
echo $content;

see https://wordpress.stackexchange.com/a/12168/152624

Okay folks:

https://github.com/littlebizzy/hovercraft/blob/master/archive-forum.php

HoverCraft has a solution for this now.

Simply make sure you have a page with the slug “forum” a.k.a. /forum/ and the theme will automatically use the title and post content for the bbPress forum index. We might add the option later to choose the page you want to use in the Customizer, but for now it seems logical that most sites will use /forum/ for the bbPress index.