I would say blocking proxies would be one of the best ideas, they're really annoying when you have spammers on your site.
I guess it'd be bad to have it as a core feature.
Let owners decide what proxy or whatever IP range or IPs to block, it cannot be good as a core feature by default.
Agreed; this is best suited as a plugin. MyBB should only be concerned about the immediate security of the software.
This idea is harmful for many users such as myself who's ISP(Internet Service Provider) proxy your connection to every site you go to. So by your theory, I would be blocked from using any MyBB website because of something that is out of my control.
Also, not all spammers use a proxy to post.
This would be better as a plugin,for the reason stated above.
What I'm saying is have the OPTION to do so.
Proxy blocking is not an exact science. It's more difficult than you think. It's something MyBB team should not spend their focus on.