From a good roundup of security updates announced at WWDC last month, by Carly Page for TechCrunch:
To ensure iPhone users who don’t want to upgrade to iOS 15
straight away are up to date with security updates, Apple is going
to start decoupling patches from feature updates. When iOS 15
lands later this year, users will be given the option to update to
the latest version of iOS or to stick with iOS 14 and simply
install the latest security fixes.
“iOS now offers a choice between two software update versions in
the Settings app,” Apple explains (via MacRumors). “You can
update to the latest version of iOS 15 as soon as it’s released
for the latest features and most complete set of security updates.
Or continue on iOS 14 and still get important security updates
until you’re ready to upgrade to the next major version.”