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.
You can add
about:reader?url=
before any URL to enable it on any website.
Omg thank you so much! I never knew this. Thank you so much for sharing this!
Only on desktop? Does not work on Firefox 98 on Android.
Yeah, only on desktop.
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