Allow altering of CAPTCHA from other modules

@AdamPS: Isn't it possible to use hook_form_alter instead for this? Does this need an extra hook?

No unfortunately not. When hook_form_alter() is called, form array contains the result of captcha_form_alter(), which looks something like this:

$captcha_element = [
          '#type' => 'captcha',
          '#captcha_type' => $captcha_point->getCaptchaType(),
        ];

The part we need to alter only gets created very late, during the final render stage.

#3314766: [2.x] Improve the CAPTCHA form markup and use twig files for more flexibility, where possible is a good idea and I look forward to it in 2.x. For the moment we only have 1.x so it would be great to have a solution that works now, thanks.

Great thanks.

One thing I'm not sure about is, if we'd also may need a way to alter the $info prior to the captcha method invocation. What do you think?

Beg pardon I don't understand what you mean by "captcha method invocation". If you are talking about
\Drupal::moduleHandler()->invoke($captcha_type_module, 'captcha' then this function doesn't actually get passed the $info, so it's not needed.

Would be great to have a backport to 1.x please seeing as 2.x is not yet ready for use in production. It seems like a totally safe fix.

Thanks here is the MR.

Great thanks