Bitcointalk.org is a forum website and not a deep web, deep web is like a underground market which you can find controversial files, illegal stuffs like porn, guns and drugs. This is a clean website and you can't compare this site to deep web because bitcointalk is created for discussing different things and not focusing on being underground.
You are right, and if it will be a darknet website. We all should use TOR browser to access in this site which it's wrong, because it's not hidden site and situated in the normal internet. Everybody can access to this site.
Also, bitcointalk.org is a big forum which based on bitcoin in general and bitcoin is not illegal as i think. So everyone can access here without any problems.