I poked around a bit, and didn’t find any direct info on how to do this, so here’s the results of my research.
If you have a WordPress.com-hosted blog (like this one), and you use categories, then the RSS feed is:
http://<domain>/category/<category name>/feed/
For example, the RSS feed for things I’ve categorized as being about “Nevada City” is https://ken-blog.krugler.org/category/nevada-city/feed/