We are not sure when it went but we have seen people complaining so it has to be quite new that it was not readily available for use as recently as February.
Silence your telephone via Google Home with new Assistant command was written from the wonderful team at Android Authorities .

Back in November this past year, Google announced a selection of new attributes for the Assistant on Home devices and telephones that included routine integration to the Android clock program among other developments. A new control that would let you enable Do Not Disturb in your phone via a Google Home speaker has been said to be”coming soon” — now it is here.