If you don't have Moodle configured to use reCAPTCHA you are still able to add captcha input fields in feedback activities. However, only once someone tries to fill out the feedback form does the message appear: "To use reCAPTCHA you must get an API key from https://www.google.com/recaptcha/admin". Since the captcha is a required field the user needs to fill it out to submit the feedback form. But since captcha isn't configured the user can't fill it out. So in this case the feedback form can't be submitted no matter what.
Steps to reproduce:
1. Make sure captchas are not configured for your Moodle instance.
2. As a manager create a feedback activity.
3. Add a captcha input field.
=> You see no warning that says captchas aren't configured.
4. Add a simple text input field.
5. As a student open the feedback activity.
=> You see the error/warning that captchas aren't configured.
6. Enter something in the text field.
7. Try to submit the feedback form.
=> You can't.
I'm not sure what the best solution would be but I wish there was some way for setups in which no captcha is used to circumvent this problem. Maybe an admin setting to disable certain feedback input field types site-wide? Maybe automatically disable captcha in feedback form creation if it's not configured (and perhaps print some message that you can see with debugging enabled)? Or at least indicate to the person creating the feedback form that captchas don't work at the moment (and that a form containing them can't be submitted)?