Is there a template tag that I can use in say main_header.html to get the link to the current page? I'm using keyword rich URL addon. I just want to use that for <link rel="canonical"... tag.

You may try to use something like this in bb_plugins.php:

then paste {$currentUrl} in the place you want.

There might be some variations with the $_SERVER variables, you may check in PHP manual for more - may be another combination would work for you.

Thanks Paul! Is there no direct (or combination) tag for keyword rich urls? Previously we could get URLs using: https://www.minibb.com/forums/5_{$topic}_1.html

In the latest miniBB series, mod_rewrite functions are separated to be standalone (and possibly modified).

Pay attention at bb_functions.php -> addForumURLPage, addTopicURLPage functions etc.

Using these functions and knowing some certain params, it's possible to build the original forum/topic URL.