TL;DR -- Categorical unsubscribe command in Tools menu prompts you for a category, and then unsubs you from all the feeds in that category.
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I took all my podcasts and turned them into a reading list, now part of the FeedCorps collection. Screen shot.
Having done this, I had the same problem @cagrimmett had a couple of weeks ago when he was putting together his lists, now that I have a list, I want to unsub from all the feeds I created the list from.
We've talked about this, and even prototyped some possible approaches for it, and gave up, but this time around the solution was totally clear -- we need a command that unsubs from a category. At first I didn't get how specific the problem was, it wasn't that I needed a mass way to edit categories, I needed a way to unsub from a category. So that's the command I made. Took less than an hour beginning to end.
TL;DR -- Categorical unsubscribe command in Tools menu prompts you for a category, and then unsubs you from all the feeds in that category.
The story that goes with this