Long live ReCaptcha v1!
Thursday, April 5. 2018
Ok. It's dead! It won't live long.
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:

Now that the above deadline is gone, I had to upgrade S9y ReCaptcha plugin from git-repo https://github.com/s9y/additional_plugins/tree/master/serendipity_event_recaptcha. There is no released version having that plugin yet.
Now comments display the v2-style:

To get that running, I simply got the subdirectory of plugins/serendipity_event_recaptcha
with the content from Github and went for settings:

I just filled in the new API-keys from https://www.google.com/recaptcha and done! Working! Easy as pie.
Update 5th April 2018:
Today, I found out that Spartacus has ReCaptcha v2 plugin available to S9y users. No need to go the manual installation path.
Ok. It's dead! It won't live long.
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:
Now that the above deadline is gone, I had to upgrade S9y ReCaptcha plugin from git-repo https://github.com/s9y/additional_plugins/tree/master/serendipity_event_recaptcha. There is no released version having that plugin yet.
Now comments display the v2-style:
To get that running, I simply got the subdirectory of plugins/serendipity_event_recaptcha
with the content from Github and went for settings:
I just filled in the new API-keys from https://www.google.com/recaptcha and done! Working! Easy as pie.
Update 5th April 2018:
Today, I found out that Spartacus has ReCaptcha v2 plugin available to S9y users. No need to go the manual installation path.