Until recently, it was possible to get the monthly top posts for, say, /r/technology using the feed URL https://old.reddit.com/r/technology/top/.rss?sort=top&t=month.
Downloading the feed with wget "https://old.reddit.com/r/technology/top/.rss?sort=top&t=month" -O technology.rss and inspecting the file reveals that the feed indeed contains the top posts as shown when accessing https://old.reddit.com/r/technology/top/?sort=top&t=month in a browser.
However, when I put the feed URL into NewsBlur, I get posts that seem to come from the first few pages of the standard “hot” view of /r/technology. The dates of the posts on NewsBlur also show that most of them are only a few hours old.
After just two or three days, I now have more than 1,100 new posts in all my Reddit feeds, when there should really only be 100 to 200 new posts in total.