In response to blip #133850
I don't think it would be, the only place that seems to actually contain the forum's index number is part of the reply box way at the bottom of the page. even if there were possible to display the text it'd be a pseudo element so you wouldn't be able to select the text to copy it or anything.
you can get the individual forum post IDs to display at the bottom of each one with:
.forum-post::before { content: "\a forum #" attr(data-forum-post-id); }
but agian, it's not super useful since you can't select the text.