I have some needs
Does a read only deadbolt exist, or are all smart locks motor equipped to allow them to be opened remotely?
Building on Helmar's answer I would suggest a DIY solution: a miniaturized reed switch and a magnetized / magnet attached deadbolt.
Check if there is place for a miniaturized reed switch within the strike plate with the deadbolt fully extended. Those switches come in packages as small as 2 mm diamater and about 10..15 mm long. Since they actually come with a glass body you need to make sure the deadbolt does not actually pushes the switch. Depending on the design of the strike plate and door frame it might be possible to increase the available space (not compromising the mechanical strength of the strike plate though).
Check if the deadbolt can be magnetized, i.e. following the esoteric procedures as outlined here "Rubbing the Metal with a Strong Magnet". Test if the deadbolt actually actuates the switch if properly aligned. Alternative if magnetizing fails: superglue a magnet to the deadbolt's tip.
Fixate the reed switch someplace inside the strike plate and run two wires to an "IoT"-like microcontroller, e.g. an ESP8266 or similar.
Total cost < 5 $, but might well take an afternoon or two to build.
Unfortunately the usual smart locks on the market are all—to my knowledge—opening your door as well. As Aurora pointed out a lot of those are currently not allowing voice activated unlocking via Google Home or Amazon Alexa. Not every lock is that smart though.
However, there's good news too. Read-only deadbolts are nothing new. Every security system has those and you can buy even the parts quite easily, e.g. here. The tricky part will be to integrate it with Google Home. I guess in some way you'll need to use either a quite expensive security solution and make sure it has a proper API or build it yourself. If you build it yourself I'd suggest something along the lines of what I linked. You can decide if you use some sort wired solution (e.g. 1-Wire to a Raspberry for example. The wireless solution would probably along the lines of a small WiFi enabled controller (maybe this one).
So after reading Helmar and Ghanima's excellent answers, I doubled down and did some research. After a little bit of work and research I came up with the following solution. Images are available here, in an Imgur album.
Purchase a flush mount reed switch and a z-wave dry contact sensor from Amazon
Drill a hole in the back of the deadbolt cavity to place the flush mount reed sensor
Pull the wires through the side of the door casing
Drill a small cavity in the deadbolt itself and place a magnet in it (I used a bucky ball)
Wire the reed switch to the dry contact sensor
Bang the door casing back together
Use packing tape or glue to secure magnet in deadbolt.
