"Action required: reCAPTCHA v1 API shutdown on March 31, 2018"

We are running recaptcha 7.x-2.2 on two different websites, and have gotten emails from Google with this subject line, and the following text:

"Dear Webmaster,

You are receiving this email because you are registered as a website administrator using reCAPTCHA, and your website is still using reCAPTCHA v1, which will be turned off on March 31, 2018.

We announced the reCAPTCHA v1 deprecation in May 2016. Starting in November 2017, a small percentage of reCAPTCHA v1 traffic will begin to show a notice informing users that the old API will soon be retired. Any calls to the v1 API will not work after March 31, 2018.

To ensure continued functionality, you’ll need to update your website to a current version of reCAPTCHA. You can learn more about reCAPTCHA v2, Invisible reCAPTCHA and reCAPTCHA Android API in our Developer’s Guide. The new APIs are simple to implement and will streamline the captcha experience for your users. If you need help, you can engage in the reCAPTCHA Google Developer Group or post to Stack Overflow with the ‘recaptcha’ tag.

We hope that your upgrade will be seamless, and we’re confident you’ll be happy with the results."

Why would Google detect use of recaptcha v1? Thanks for your help.