For some time now, Apple has been warning iPhone and iPad users that legacy 32-bit apps may slow down their devices, but with the recent release of iOS 10.3, Apple has escalated things by making it clear that the end is nigh.
If you've seen the warning shown below in relation to an app you use, then that app is a 32-bit app and Apple is getting ready to withdraw support (quite possibly as soon as September, when iOS 11 is released).
You can also check for app compatibility using the built-in checker tool. Go to Settings > General > About > Applications:
How many apps are affected? One study suggests the number could be around 187,000 apps, or some 8 percent of the 2.4 million apps in Apple's App Store, with apps in the games, education, and entertainment categories being the hardest hit.
Best iPhone or iPad apps (March 2017)
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