A little while back the signature "hey Google" Assistant commands stopped working on Wear OS watches. Well, that's not completely true: a few months ago it stopped working, and it took that long for enough people to notice that Google said it would do something about it. Based on a few user tests, that appears to be resolved, at least for some devices.
On the Wear OS subreddit, a user with an Oppo Watch took a short video of the command functioning once a gain. A few others said theirs was working too, notably on some Fossil Gen 5 and Gen 3 watches. Many others say it still isn't functioning for them. We tried it on an Oppo Watch with no luck.
For what it's worth, at least one poster who reported Hey Google as functional said they were using the 18.104.22.168 version of the Google app, which doesn't seem to be widely available yet. We can hope that, for those people who still care about this capability (and haven't turned it off to save battery life anyway), it comes back soon.