According to this post, I thought the new recaptcha API was used network-wide. Though today, when going to SEDE (which for whatever reasons never recognized me automatically), I got this message instead of a usable captcha image.
Please direct siteowner to g.co/recaptcha/upgrade
(Doing a second try doesn't show this message anymore, and the recaptcha v1 works as intended.)
According to the linked page, the reCAPTCHA v1 API will stop working on March 31, 2018:
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.
Most websites around the world have already switched to reCAPTCHA v2, where humans pass effortlessly. With our advanced risk analysis engine, reCAPTCHA v2 can effectively separate humans from bots and always stay ahead of the attackers.
If your site still uses reCAPTCHA v1, please upgrade to reCAPTCHA v2 immediately. We no longer support reCAPTCHA v1. Any calls to the v1 API will not work after March 31, 2018. Please register a new key and upgrade to v2.
I'm really hoping this will not take 6 to 8 weeks to fix, because we've only got three and a half weeks left!