Apple's iOS 11 mobile operating system will not be available for the iPhone 5 and 5C or the iPad 4 when it is released in the autumn.
It means those with the older devices will no longer receive software or security updates.
The iPhone 5S and newer devices will receive the upgrade but some older apps will no longer work afterwards.
The news is the result of Apple's decision to end support for devices and apps using 32-bit processors.
Apple has been making devices with 64-bit processors rather than 32-bit processors since 2013, when it introduced them with the iPhone 5S and the iPad Air.
Apps that only run in 32-bit will not show up in search results in the new version of the App Store, or be available from the Purchased tab if they have been downloaded previously.
'Warning for years'
A 64-bit processor can handle twice as much data at once as a 32-bit processor, which can help make it faster in use.
"Apple has been warning of this migration to its 64-bit hardware for a number of years, but this news will still undoubtedly blindside a number of its customers," Ernest Doku, a telecoms expert at uSwitch.com told the BBC.
"Most apps from the last four years or so should be compatible, unless they haven't been updated since 2015."
When Apple released iOS 10.1 in October 2016, users trying to open a 32-bit app on an updated device would receive a warning message telling them that it may slow down their iPhone.
The update to iOS 10.3 included a tool to detect apps that cannot run in 64-bit mode.
Affected apps will be listed under Settings > General > About > Applications > App Compatibility.
All new apps and updates to existing apps have had to support 64-bit since June 2015, so any that have been updated in the past two years should be unaffected.
But some popular games and educational apps have not been updated since the 64-bit requirement was introduced.
They include titles from Fisher Price and WeeWorld, creators of the WeeMe avatar app.
Apple is also starting to phase out 32-bit support for the Mac, telling developers at WWDC that Mac OS High Sierra would be the last to have 32-bit capability "without compromises".
From June 2018, all new apps submitted to the Mac app store will need to support 64-bit, and from June 2018 updates to existing apps must also be 64-bit compatible.