Stack Overflow seems to have abandoned us iOS users! Look:
What has happened?!
- App Version: 188.8.131.52
- Device: iPhone 5s (Global)
- OS Version: Version 10.3.2 (Build 14F89)
Never mind, I figured it out.
The "All Sites" page on the iOS app removes any sites that you have pinned to your sidebar. As you can see, Stack Overflow is pinned to my sidebar:
I only noticed this when I realised that none of my sites that were pinned showed up in the list of sites. So this isn't really a bug!