how to protect WordPress RSS Feed from “Feed Fetcher”

You’ll need to look in your logs to find what User-Agent string Feed Fetcher is using, and then you can block it using that, e.g.

SetEnvIfNoCase User-Agent "Firefox/70.0" blockua
Deny from env=blockua

By adding the above to my .htaccess file, I was able to block the Firefox (version 70.0) but Chrome still had access.

Obviously, you need to change the string “Firefox/70.0” to whatever the Feed Fetcher user-agent string is, and most likely you don’t want the entire string, but just a part that look unique to that piece of software and won’t accidentally block something else.