Archiving Yahoo<i>!</i> groups
I found, and have used, the script at the end of this post to download all posts from the groups I manage. (Fortunately, we'd used them just to swap messages; we didn't have any significant files uploaded to Yahoo!.) The posts are provided as HTML spaghetti; I am working on code to convert the posts to a usable form. As you'll see, it's a work in progress... but the important thing is to grab your data before Yahoo! deletes it "to serve you better" / "to increase shareholder value" / "to improve the customer experience". The script will grab that data for you.
Many people are moving to groups.io. I'm using GNU Mailman on my own server. groups.io appears to be a decent place at present, but then again, Yahoo! was decent for some time.