Please take a look at Samsung's Android Security Update page (https://security.samsungmobile.com/workScope.smsb). It shows February 2018, March 2018, and April 2018 updates available for current models including the Galaxy S9 and S9+:
Introduction to Samsung Android Security Updates
At Samsung, we take security and privacy issues very seriously and we are doing our best to respond as quickly as possible.
Securing your device and maintaining the trust you place in us is our top priority.
Samsung releases monthly and quarterly security updates on selected Samsung devices listed below.
Monthly and quarterly security updates will include patches for Android OS related security issues released by Google, as well as, patches for Samsung-specific security issues.