Apple’s biggest surprise at this year’s WWDC was, without a doubt, iOS 15. This new system brought a wide range of news and improvements, making it even better and more adapted to users’ needs.

It is expected that Apple will want all users to upgrade as quickly as possible, as they always do. This year, and with iOS 15, something may have changed in that stream. It will allow you to keep the previous version and will release continuous updates to iOS 14.

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A major change in Apple's ecosystem
iOS 14 certification numbers have reached near-perfect numbers recently.  Install growth has been rapid and the latest official and upcoming data from Apple shows that this is already present in 90% of iPhones.
Given this scenario, Apple's bet was now expected to change to iOS 15 and all of its new features.  Expected in September, it will only make room for security updates and point fixes in the previous version, paving the way for the update.
iOS 14 will be preserved with updates

iOS now offers a choice between two versions of software updates in the Settings app.  You can update to the latest iOS 15 as soon as it is released with the latest features and a full set of security updates.  Or stay on iOS 14 and get important security updates until you're ready to upgrade to the next major release.

The truth is according to the information that Apple it's spreadingThe update to the new version of iOS may be delayed for a longer time.  There will be no pressure on users to install as iOS 14 support will be maintained for longer than expected.
In the normal scenario, this support may be lost after the new version arrives, which is not going to happen now.  This version will continue to receive security updates, which will keep this system usable and at the same time with the necessary security protection.
No stress to move to iOS 15
This is a novelty that many can enjoy.  On the one hand, for those who want to keep the current version until they are sure of stability, and on the other hand for those who will have to stay in that version, and who can thus reach the resolution of security issues.
With this change, Apple unlocks the range of supported systems and updates that you must manage.  However, it does open the door to some fragmentation, something that turns out to be the biggest problem with Android.