Set default RSS reader
I found the launching of Apple News to deal with RSS/ATOM feeds a bit of overkill; I don't like the News app. Period.
I installed the open source Vienna feedreader, and then in the Vienna preferences, I found that I could select between Apple News, Vienna, and Feedly (a cloud-based feed reader) as options for preferred app for RSS feeds. So, apparently, the answer to the original question of how to "Set default RSS reader in Mojave" (link: https://discussions.apple.com/thread/250143720)... is to first install an alternative RSS reader besides News, and then set THAT to the preferred choice, rather than Apple News.
I'm actually running Catalina now, and everything seems to work fine. So, I guess, to override Apple News, you need another feed reader on your local system. Hope this helps.
.LAG