not sure if a sitemap.xml really adds “more” pages of this forum to the search engines; i think this is true mainly for very unsorted, large sites with lots of duplicate content etc. the structure of the pages in this forum seems to be pretty straight-forward.
however, afaik, a sitemap.xml may also help to rank pages better.
so, i’d say, if it is really simple and does not result in maintainance, we can go for it. in fact, we also have a sitemap.xml for delta.chat - https://delta.chat/sitemap.xml - mainly because it was cheap.
but if installing is more than a few minutes, few lines of code or so, i’d say it is not worth the effort just now.
You’re right, this forum is already search-able through search-engines, but the rank isn’t high enough yet, I need to add site:support.delta.chat to get enough forum results on the first page of the results. Maybe a site-map could help a bit with SEO.