As z-component pointed out, threads only get locked/deleted if they violate the guidelines. Note, sometimes that is due to the content of the original post in the thread, and sometimes due to out-of-control conduct of people posting replies in the thread. We also may lock some threads because they have reached an impasse and are no longer going anywhere other than repeating the same points ad nauseum.
The forum global guidelines are located right here in the feedback and announcements forum in a sticky thread (currently immediately above this one). This will tell you what discussions we expressly forbid on the site. It is worth noting that we are in the process of revising the guidelines slightly to clarify a few gray areas (for example, in light of jostpuur's post just a few threads down, we realized there was no statement about disallowing conspiracy theories, although this is listed within the S&D forum where conspiracy theories usually land, so we are rewriting them to explicitly state this).