This is an important concept for people to understand.
All aspects of security (policing, counter-terrorism, cybersecurity, etc) are all fundamentally impossible to prove how effective measures taken are. Usually the best we can do is use a proxy measure or test pre/post.
There would be media reports and court cases to point to if any of these measures were effective. As a reminder, virtually every school shooter was monitored or on a watchlist.
All aspects of security (policing, counter-terrorism, cybersecurity, etc) are all fundamentally impossible to prove how effective measures taken are. Usually the best we can do is use a proxy measure or test pre/post.