Google at some time announced the following for the old reCaptcha v1 version:

So we decided that it's time to stop supporting the old version of reCAPTCHA, which means:

We won't develop any new features for reCAPTCHA v1.
Any new key registered on reCAPTCHA will be only valid for v2.
If your site still uses reCAPTCHA v1, it will continue to work, but please expect that it may become less functional. We encourage you to register a new key and upgrade to v2 soon.

Source: https://developers.google.com/recaptcha/docs/faq

ConfirmEdit still supports the old reCaptcha version, but it seems to doesn't make sense anymore, as Google will not support the version in the near future (I suppose). So we should find a way of either:

• Depreacte the old reCaptcha in Version 1.28 of MediaWiki and remove it in the upcoming 1.29 release
• Deprecate the old reCaptcha in the upcoming Version 1.29 or MediaWiki and remove it:
  • in MediaWiki 1.30
  • When Google doesn't support the old reCaptcha anymore

I'll send a notification about this task to wikitech-l and probably some other lists (where mostly third-party users should be subscribed to, as they're the only ones using this module) asking for opinions. If no-one seems to be interested in writing something, I'll choose one way and upload a change for it :P