Works great with reCAPTCHA v2 checkbox!

reCAPTCHA for TYPO3 EXT:form

You can either choose to define your reCAPTCHA api Keys with typoscript, or as environment variables.

Typoscript setup

Check out the typoscript constants file for it: formrecaptcha/Configuration/TypoScript/constants.typoscript

Make sure you add constants for publicKey and privateKey.
You could also change apiScript and verificationServer but that is not necessary.

Environment variables

I suggest to use DotEnv Conntector for this composer req vlucas/phpdotenv

Define your reCAPTCHA api keys as environment variables:
GOOGLE_RECAPTCHA_PUBLIC_KEY and GOOGLE_RECAPTCHA_PRIVATE_KEY
You could also set GOOGLE_RECAPTCHA_API_SCRIPT and GOOGLE_RECAPTCHA_VERIFICATION_SERVER but that is not necessary.