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> This does not work for sophisticated bots (never met one)

The basic prerogative of a sophisticated bot is to ensure you believe that.

Having written a variety of sophisticated bots - some from pre-existing libraries, others from scratch; for specific websites and for general purpose - I'm reasonably confident most people who think they've never seen a sophisticated bot are mistaken.

I agree with you that anything more sophisticated or bespoke than a mass-spam bot is rare. But rare things happen often to most websites with nontrivial traffic. The types of bots with the most funding and skill behind them are the ones which don't try to spam anything on a website at all.

But this is good enough in many cases. It's intended to reduce the volume of input from a form in order to prevent overloading human resources at the next level of filtering. If one or two bots per week get through, it isn't a big deal. Not only are the more sophisticated bots less common, they also tend to not send a bunch of messages.

So if you have a contact form, this simple method may reduce the spam content to a low enough level that the effort to implement some type of third-party service is not necessary. There is not enough incentive for someone to target the form directly.

In addition, the captcha service may actually deter real submissions, whereas this is completely invisible to non-bots.

If you have a form that has a greater incentive for bots to abuse, then you need something more sophisticated.

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