A private GDPR-compliant option is ShinyFriendlyCaptcha, with documentation at https://mhanf.github.io/ShinyFriendlyCaptcha/index.html. It does require an account with FriendlyCaptcha (https://friendlycaptcha.com/), but there is a free non-commercial option. It's still experimental, but quite a bit more recent (2023) than CAPCHAv2 and v3.