The Reader Mode is currently available for Chrome on Android only with name ‘simplified view’, but not for other platforms such as Windows, Mac, Linux, and Chrome OS. On January 28, 2019, a Google engineer created a bug to implement “Reader mode for the desktop” saying “The simplified view offered by Chrome on Android should also be available on desktop platforms”. While that bug is for tracking work needs to done for feature, the associated Reader Mode bug for desktop Chrome could be this.

Google to bring simplified View from Android to desktop Chrome
Firefox, Microsoft Edge, Safari and Vivaldi allows users to view and read content without any clutter such as popups, menu, advertisements, sidebar and other distracting elements on the supported webpage by showing a button on toolbar/address bar when articles with long text appear, which when clicked, users enter so-called Reader Mode.

Four years back, Chrome has offered a flag called Dom Distiller, which when enabled distills the content of the page, thus provides a better reading experience to the users. The flag like other, over the time has vanished but available on Android for Chrome behind flags page. Sometime back, Google has shipped the Simplified View for Android version of Chrome 65.
distilled page contents Chrome desktop
For uninitiated, here is how you can enable reader mode on the Android version of Chrome,
Launch Chrome, click on Settings >Accessibility,> Simplified view for web pages.
Chromium developers promised to offer Reader Mode, we don’t know when it will be available, we’ll let you know when they offer the feature for testing. So stay tuned for further updates.