I have a change up [1] to deprecate the backward compatibility code in the ResourceTracker for virt drivers that do not implement the update_provider_tree interface. Starting in Train you'll get a warning in the nova-compute logs every time the update_available_resource periodic runs (we could only log on startup, that's up for debate in the change) until the driver is updated and in the U release we'll drop that compatibility code.
The only out-of-tree virt driver I checked was nova-lxd and it does *not* implement update_provider_tree so for those that care about that driver you should implement the interface. The patch has a link to the reference docs on implementing that interface for your driver (it's pretty simple).
[1] https://review.opendev.org/#/c/667442/