They announce when the phone is EOL. Before that, security updates are fairly frequent. There is a mailing list for Security updates. You don't need to be on it to get the updates (automatically), but the emails tell you when it's available and what it covers. A short excerpt. APPLE-SA-2020-12-14-2 iOS 12.5I didn't know that! How long does Apple support security updates for their phones?
iOS 12.5 addresses the following issues. Information about
the security content is also available at
About the security content of iOS 12.5.
Security
Available for: iPhone 5s, iPhone 6, iPhone 6 Plus, iPad Air, iPad
mini 2, iPad mini 3, and iPod touch (6th generation)
Impact: Unauthorized code execution may lead to an authentication
policy violation
Description: This issue was addressed with improved checks.
CVE-2020-27951: Apple