^^^^ This is my frustration. That I understand that within the apps that I use for work I understand not being able to share to an app like messages. That being said I should be able to share pictures to my messages apps as it has worked in iOS9 and iOS 10.
I think it's clearly an issue with new settings in iOS 11, or MDM profiles being outdated and needing updated to work with iOS 11. This also worked for me on iOS 10 but doesn't on iOS 11. There was the prior issue with Apple Store (Apple Music, App Store, etc.) not working on Beta 1 and 2 due to MDM profiles, so Apple must have overhauled how MDM profiles work on iOS 11.
Even if the share sheet disables sharing photos, you can still send photos from going to your messages and selecting photos from there.. so the profile isn't working as intended, if it's job is to prevent sharing of photos through messages. It doesn't make sense iOS 11 would newly set this policy, but rather the MDM profile should be doing that.
tl;dr GM of iOS 11 should fix this, or your MDM profile (once your MDM issuer confirms compatibility with iOS 11 later on) will be updated to remove these compatibility issues.