Does adb backup include contacts? Does the command:

adb backup -all -f backup.ab


include contacts in the backup?
No, it would not backup Contacts. Yes, it would backup Contacts.

Confused, eh!


  Yes, it would backup Contacts.


See this question: Backup/Restore SMS/MMS via ADB on a non-rooted device? The author of it was able to backup contacts. It was later revealed that the device on which the backup was performed ran Android 4.1.2. It was a bizarre revelation for me that the backup succeeded. 


  No, it would not backup Contacts.


As per the Manifest mentioned here for default Contacts Provider app in Android versions (in series), for Jelly Bean and above Android versions the ADB backup for Contacts Provider/Storage would be denied by system, and hence your contacts wouldn't be backed up.

It's a mere flag android:allowBackup="false" that's the cause of backup denial. See my answers here and here to know more about that flag.