That seems to suprise few people. I know it did surprise me.
Google has had this info in their website for couple years already:
What happens to reCAPTCHA v1?
Any calls to the v1 API will not work after March 31, 2018.
Starting in November 2017, a percentage of reCAPTCHA v1 traffic will begin to
show a notice informing users that the old API will soon be retired.
Yup. This blog showed information like this on comments: