Proton has opened sourced everything so far and I would expect them to do that here. They have whole pages written on why they open source everything and why that helps privacy.
How exactly do you have an open source captcha. If it were open source any one could read up on exactly how it verifies someone as a genuine human and replicate it.
Nothing is mentioned other than "proprietary system". Probably meaning that both ends are closed source. I don't see how I can verify whether it respects my privacy or not. I don't see a reason to implement this instead of mCAPTCHA, which is fully FOSS.