Unable to add restricted profile on Xperia Z2 tablet
I turns out that LTE versions of the Xperia tablets (tablets with a SIM slot) are treated as if they are a phone, so the option is disabled:

https://talk.sonymobile.com/t5/Xperia-Z3-Tablet-Compact/missing-restricred-user-profiles-on-android-5-0-2/m-p/956279#M455

I opened an issue at Google:

https://code.google.com/p/android/issues/detail?id=180281