Apple confirmed that users will get security update for previous iOS release for some amount of time and then should proceed with update to iOS 15.
If you're using iOS or iPadOS 14.5 or later, you might now see the option to choose between two software update versions. This option allows you to choose between updating to the latest version of iOS or iPadOS 15 as soon as it's released, or continuing on iOS or iPadOS 14 while still getting important security updates for a period of time.Apple Support
Currently, it is expected that users will need to upgrade to iOS 15.
Apple Support: Update your iPhone, iPad, or iPod touch