As of 2021, the Planday app will no longer be supported for devices using iOS 12 or prior versions.
This means that you will not be able to update the Planday app if you are using a device with iOS 12 (or lower) as it will no longer be supported.
You still will be able to use the latest version available for that device, for as long as needed.
Updates to include the specific versions of the Planday App, to follow shortly.
Considering that a device is using the latest iOS version available, the list of devices supported is:
iPhone SE (2nd generation)
iPhone 11 Pro Max
iPhone 11 Pro
iPhone 11
iPhone XS
iPhone XS Max
iPhone XR
iPhone X
iPhone 8
iPhone 8 Plus
iPhone 7
iPhone 7 Plus
iPhone 6S
iPhone 6S Plus
iPhone SE
iPod Touch (7th generation)
As of 2021, if you're using any following devices you will no longer be able to update the Planday apps:
iPhone 5S
iPhone 6
iPhone 6 Plus
iPod Touch (6th generation)