Well this is not the first time Youtube changed their policies. Earlier Youtube had stricken their policies following the Logan Paul worldwide controversy for posting a suicide video of a victim in a Japan forest.
This time due to a recent shootings in the U.S. Youtube have again change their policies in order to prevent such incidents from happening in the future.
So now Youtube will no longer be allowing videos of buying and selling of firearms and the accessories related to it on their platform, as well as stop the sharing links of sites which are redirecting to firearms selling sites.
Youtube have already started removing videos tutorials of installing/making of firearms and the accessories related to it.