Google has quietly announced that on September 30th, Book on Google for flights will be discontinued in most countries or regions outside the United States.
I am not sure why this is the case, maybe because there are just too many regulatory and monopolistic issues with the service outside of the US, and the ROI to deal with it with lawyers is not worth it. I am just guessing...
"Reservations made before this date won't be impacted," Google added.
But if you have questions about your current Book on Google reservation, you can still contact the booking partner in your reservation confirmation, as well as Google.
This does not seem to impact services within the United States.
Forum discussion at Twitter.
Update: A Google spokesperson sent me this statement, "Over the next 12 months, we plan to phase out the Book on Google feature for Flights. We originally offered this functionality to give people a simpler way to buy their tickets and to help our partner airlines and OTAs receive more bookings. However, we’ve found over time that people actually want to book directly on partner websites, and we always strive to meet user preferences whenever possible."
Also, after March 31, 2023, Google will disable Book on Google for US Google Flights results as well.