That is not a flag for me in version 79.0.3945.130 on Mac. What OS / version of chrome are you using?
Huh, I just updated to that version and I no longer have the flag either, guess they got rid of it... One more reason to primarily use firefox.
Looks like there are extensions in the chrome store that essentially do the same thing though.
Shows only first paragraph I am afraid. I am on latest stable version of Firefox on macOS.
Firefox has this feature enabled by default and displays an icon in the address bar that you can click to reload the page in reader mode.