Q:

How did the WhatsApp support team knows I have a rooted phone?

I emailed WhatsApp support team, via the built in Help feature. 
It automatically generated a ticket which I emailed in. 
Seems that they have "detected" that my phone is rooted, and they do not support that. How did they "detect" that my device is rooted? Is it from the Build? The Kernel? The Diagnostic Codes? Or the missing permissions? How will a "passable" debug email will look like? Will apps that "hide my root" fix this debug email to be passable?
Here is my email that was sent, (removing personal info):
--Support Info--
Debug info: <REMOVED PHONE NUMBER>
Description: 2.17.351
Version: 2.17.351
LC: US
LG: en
Context: settings/about
Carrier: T-Mobile
Manufacturer: OnePlus
Model: ONE A2005
OS: 7.1.2
Socket Conn: UP
Radio MCC-MNC: 310-260
SIM MCC-MNC: 310-260
Free Space Built-In: 34432339968 (32.07 GB)
Free Space Removable: 34432339968 (32.07 GB)
FAQ Results Returned: 10
FAQ Results Read: 0
Smb count: 0
Ent count: 0
CCode: <REMOVED PHONE NUMBER>
Target: release
Distribution: play
Product: OnePlus2
Device: OnePlus2
Build: OnePlus2-userdebug 7.1.2 NJH47F b4eb8634d4
Board: MSM8994
Kernel: 3.10.107-perf+ #1 SMP PREEMPT Sat Sep 30 23:53:43 PDT 2017
Connection: M.O.B.I.L.E. (L.T.E.)
Device ISO8601: <REMOVED TIMESTAMP>
Phone Type: G.S.M.
Network Type: L.T.E.
Missing Permissions: android.permission.ACCESS_COARSE_LOCATION, android.permission.ACCESS_FINE_LOCATION, android.permission.CAMERA, android.permission.INSTALL_SHORTCUT, android.permission.READ_PHONE_STATE, android.permission.RECORD_AUDIO, android.permission.WRITE_SETTINGS, com.sec.android.provider.badge.permission.READ, com.sec.android.provider.badge.permission.WRITE, com.htc.launcher.permission.READ_SETTINGS, com.htc.launcher.permission.UPDATE_SHORTCUT, com.sonyericsson.home.permission.BROADCAST_BADGE, com.sonymobile.home.permission.PROVIDER_INSERT_BADGE, com.huawei.android.launcher.permission.READ_SETTINGS, com.huawei.android.launcher.permission.WRITE_SETTINGS, com.huawei.android.launcher.permission.CHANGE_BADGE
Architecture: armv8l
Diagnostic Codes: <REMOVED CODE>
Network metered: 100:true
Network restricted: 100:false
Data roaming: false
Tel roaming: false
ref: <REMOVED NUMBERS>
Video transcode: supported
No log file to attach.

And here is their auto response: 

Thanks for your message.
It appears that your device is rooted. We are sorry, but we don’t
  support rooted devices and don't answer tickets from rooted devices.
  We understand the reasons behind rooting a device, but please be aware
  that any new configuration that is a result of rooting may conflict
  with WhatsApp.
To add to this, rooted devices don't allow the WhatsApp security model
  to function as intended and your messages will not be protected by
  end-to-end encryption.
If you wish to receive further assistance, please remove root access
  from your device. Then, contact us again from inside the app (WhatsApp
  Settings > Help > Contact us).
Responses to this email will not be read.

A:

From your build version OnePlus2-userdebug 7.1.2 NJH47F b4eb8634d4 , one can easily say that you are using a rooted build. 

userdebug like "user" but with root access and debuggability; preferred for debugging

For more info about build type see this
In this case any root hide app will not work. You can try to edit build name along with root hide.

A:

I, as a programmer, will try to describe how the root is determined on the user's device.
Programmers should check some signs on your software which definitely show "is your device rooted" or not.
For example:
I wrote a sample app which checks a few signs in user's device.
You can see here(my repository). To be honest, I would not want my answer considered as spam.
As a programmer I should do in my application next checks:

TEST KEYS
DEV KEYS
NON RELEASE KEYS
DANGEROUS PROPS
PERMISSIVE SELINUX
SU EXISTS
SUPERUSER APK
SU BINARY
BUSYBOX BINARY
XPOSED
RESETPROP(EXPERIMENTAL)
WRONG PATH PERMISSION
HOOKS

You can download this app from google play(without any ads or analytics) or compile it yourself from the repository.

