There’s a boilerplate RSS template https://docs.expressionengine.com/latest/add-ons/rss.html#sample-rss-and-atom-templates (see link on page).
What goes into the feed is up to you because you’d normally pull the data from channels and entries that you specify in the template. To get forum entries you can use the same RSS template but populate it with the {exp:forum:topic_titles}
tag.
The old forum templates I’d sure already have a feed available, linked in the footer as I remember - it’s been a long time since I played with one.
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