On my phone it's Settings => Display => LiveDisplay => Reading mode ("Grayscale mode for long-term reading"). I can also add a reading mode toggle button to the quick settings in the notification bar.
The search box in Settings will probably find it if you search for gray or reading.
This is on LineageOS 18 (Android 11), though I recall it being mentioned in some Google events/announcements so I'm 99% sure it's a core Android feature and not something specific to LOS.