The problem is some brilliant product manager decided that the reader mode button should not be available on all websites and should only be available on those it seems to be articles.
I’ve never had reader mode not work on archive.is
Which browser do you use? I've never seen it work on archive.is in Firefox. Case in point: https://archive.is/ZPvrO
Specifically only speaking of iOS and Safari. Don’t have FF installed and have never found reader mode option in Chrome.