It is a consequence of iOS 11 being restricted to running apps written in what is known as 64-bit code. So if an app was written in the older 34-bit code it won't work.
One side-effect of this is something Apple have been warning users about for a while: they're no longer going to support anything older than an iPhone 5S (earlier phones won't receive the update at all). On the plus side (for most users, if not advertisers ), Safari will no longer be supporting auto-play videos.
To see which of your apps are at risk, go into:
Settings > General > About > Applications
I don't have a lot of apps as it is, but the only one for the chop on my phone is essentially obsolete anyway. I've noticed that most of the apps I rely on - e.g. travel apps - have been updated recently to 64-bit, although it looks like a lot of simple but popular games could be hit.
Lots of useful advice here:
http://www.theregister.co.uk/2...11_upgrade_hold_off/
Summary: try to hold off and avoid the first release of iOS 11 (there's bound to be a bug fix in a few weeks), but if you really must, make sure you take a backup first.