@ProThoughts Definitely that is an approach that's worth answering :) However, there are a few things to consider here:
1. I guess you're suggesting to upgrade to reCaptcha v3. If you make some research you'll figure out it doesn't really improve bot detection. It just makes life easier for humans and allows to better decide what to do with the bots. In other words, a successful registration in v2 will mean a successful registration in v3 (unless you tweak it up to a value in which you might lose real registrations).
I acknowledge the fact that the docs state that v3 does a better job letting spammers out by analyzing there actions in the site. However, the registration is the first step, so there shouldn't be any history in the site.
Also bare in mind that everything I said about well-known captcha solutions in my previous comment, still applies here.
Regarding adding questions, I think there already is a plugin for that? Maybe from @q2apro? Those plugins are useful to stop fully automated bots. However, they are almost of no use to bots aided by humans. The bot just needs to try to register. Read the question, look for the answer in the database, use the answer if present or send the question to the bot developer so that they can input the answer. Then that question is useless. If you think about it, the site owner had to think the question, write it, answer it and save it. The bot the developer just needed to store the answer in the database. This kind of plugins would only work if you change the question every day (and don't use any previous one, of course) and hope for the bot developer to get tired of answering your question.
Anyway, I think this approach will stop fully and naive automated bots. This is valid as long as you don't publish the code and the solution does not get popular so that it is worth breaking.
2. This will not prevent spam. This will post-process seemingly-spammer users. This is just a cleanup activity rather than a spam prevention approach. In fact, I believe there was a plugin called User Management from @qa-themes. Deleting users that match that criteria is a kind of feature that could definitely be part of it.
> I know there are some plugins like logical captcha for q2a but this feature must be part of Q2A.
I don't agree with this. For either of the approaches a plugin can be (or has already been?) developed. Furthermore, and I'm going to repeat myself again with this, but if this is part of the core, then all bots will break it. So it is even worse to have it in the core. So why do you say it must be part of Q2A?
> Adding new field on registration page will help only for few days but they will crack that also. I have seen SPAM users on such Q2A.
This is similar to item #1. You are adding a new field. Any fully-automated bot can break that. However, if you do it just for your site, the bot developer will have to create the ad-hoc solution for your site. Then it is up to them to decide if it is worth breaking it or not. Also the statistics I've shared in my previous post are still valid after 4 months of adding the custom checkbox.