The article mentions enabling Color Filters as a shortcut action via triple-clicking the power button. It just so happens that the default filter is greyscale.

If you want to play around with the other options (or permanently set greyscale), it’s under Settings —> Accessibility —> Display and Text Size —> Color Filters. Or just search Color Filters from the main Settings search bar.

Accessibility> Display & Text Size> Color Filters> On then top option is Grayscale.

Color Filters also has a true no blue mode for night.

It's convoluted for some reason...

Setting -> Accessibility Display & Text Size Colour Filters

edit: Shoulda refreshed before I commented...

accessibility -> display & text size -> Colour Filters.

Turn that on and then you can select "Greyscale" as an option. I have this mapped to the double tap the back of the phone with an accessibility shortcut.

Basically I can double tap the back to toggle greyscale mode.