Registration forum is open to anyone who can successfully submit a registration. This is true no matter what you do.
Many people incorrectly diagnose people registering by hand as you put it, "botting".
What does the new version do?
A better captcha written from the ground up based on breaking theory that is used to break captchas. That is, it has been designed specifically with intent not only to be difficult to break, but by understanding how captchas are broken, and implementing methods to break those methods.
The problem is the trust model is set to allow everyone to get the form, and then submit it. However if you change the trust model, your forum will not work.
Unfortunately a slim few have exploited the early trust models of the internet, and we have the mess we have today for it. Dealing with it involves labelling potentially innocent people guilty and denying them access for various reasons including disability, or the fact they are unfortunate enough to share an ISP with someone who is naughty. It is like branding countries for the act of a sole person in that country, it is not really fair, but that person causes so much damage that there is little you can do to stop it.
Different tweaks will work for different communities, but when you release phpBB, you are releasing something that has to appeal to everyone.
So you can stop bots by clever captcha design. But how do you stop human bot farms when you want a human to be able to submit the form? That is segmenting human with malicious (probably too strong a word for here) intent from one with honest intent. This is difficult because they are both human and faceless.
Of course the code is more secure and locked down by design than ever.
Other features include maximum registration form tries for an IP, again easily circumvented with a large rotation of proxy IPs. So it becomes a war, and the person with the most resources wins. Fortunately alot of these people have little resources and are easy to block, especially those who target small sites, blogs, and forums.
phpBB3 it becomes even more difficult to fingerprint registrations due to the removal of profile fields from the registration form, such as website, interests, and occupation that bots lick up.
So now I hope you see the enormity of the problem. If we could go back to 1999, would you?