A way to enable a reader mode has come and gone over the years, always "in testing." That's not the same as a first class feature, and it's clear they have no intention of making it available by default. Defaults matter.

In case you doubt this, here's an article from 2014 on how to enable the perpetually newly-added hidden reader mode: https://lifehacker.com/enable-the-new-hidden-reader-mode-in-...