> This is only truly necessary as a mechanism to prevent automated signups.

Only if there isn't some shared knowledge between the website and prospect.

For example some niche websites that I use have locally-based challenges such as 'how many engines can be fitted to a Boeing 747-400'* or a short mathematical algebra to solve.

Both approaches work without Javascript and without external support, just with a two-column table and a trusted group of challenge-creators.

* it's more than four