[Nexus 5X Q&A, Help & Troubleshooting] DebianShaka:
I found a reproducible way to install Kali NetHunter on a Nexus 5X (in March 2021, in case you bump on this thread in the 'future')

Do this at your own risk. While it works, it erases everything on your phone. Please learn how to backup and use fastboot and TWRP first

Scope :

Model Nexus 5X, codename bullhead

A spare phone, you will erase everything on it by following the instructions

Installation of Android 8.1.0 OREO, namely https://dl.google.com/dl/android/aosp/bullhead-opm7.181205.001-factory-5f189d84.zip

With the addition of cache.img from a 7.1 archive, namely https://dl.google.com/dl/android/aosp/bullhead-n4f26u-factory-e6420fc1.zip

Installation of SuperSU 2.82, namely SR5-SuperSU-v2.82-SR5-20171001224502.zip from https://download.chainfire.eu/1220/SuperSU/SR5-SuperSU-v2.82-SR5-20171001224502.zip/

Installation of Kali NetHunter 2021.1, namely https://images.kali.org/nethunter/nethunter-2021.1-bullhead-oreo-kalifs-full.zip

All this using fastboot and TWRP 3.5.2, namely https://dl.twrp.me/bullhead/twrp-3.5.2_9-0-bullhead.img.html

You, a fluent Linux (preferably) user with some knowledge of fastboot and adb - including the proper google USD drivers installed if on windows. You've been warned

Enough warnings. After long and painful weeks of tests I finally succeeded in installing NetHunter on Nexus 5X, the crucial point being SuperSU works whereas M.a.g.i.s.k does not (redacted to prevent search to get here)

Step 1​

Download the prerequisites, create an install directory on your pc

Unzip there bullhead-n4f26u-factory-e6420fc1.zip/cache.img

Unzip there bullhead-opm7.181205.001/image-bullhead-opm7.181205.001.zip/*.img

Using the key combo POWER + Volume Down, boot in fastboot mode.

Connect your phone to the PC/Mac/Linux

Step 2​

Here goes the (non-bricking but) destroying commands

D=YOUR_INSTALL_DIR
cd $D
fastboot format userdata \
&& fastboot format cache \
&& fastboot flash bootloader $D/bootloader-bullhead-bhz32c.img \
&& fastboot reboot-bootloader \
&& sleep 6 \
&& fastboot flash radio $D/radio-bullhead-m8994f-2.6.42.5.03.img \
&& fastboot reboot-bootloader \
&& sleep 6 \
&& fastboot erase cache \
&& fastboot flash cache $D/cache-bullhead-n4f26u.img \
&& fastboot flash boot $D/boot.img \
&& fastboot flash recovery $D/recovery.img \
&& fastboot flash vendor $D/vendor.img \
&& fastboot flash system $D/system.img \
&& fastboot boot $D/twrp-3.5.2_9-0-bullhead.img \
&& sleep 30 \
&& adb push $D/SR5-SuperSU-v2.82-SR5-20171001224502.zip /sdcard/SR5-SuperSU-v2.82-SR5-20171001224502.zip

Your phone should now be inside the TWRP gui (if necessary slide to answer "yes I want to modify the system partition")

Step 3​

From within TWRP, choose "Install"

Select SR5-SuperSU-v2.82-SR5-20171001224502.zip

Wait for the end of the install

Also run Wipe Davik/Cache

Select reboot > system

Your phone should boot within less than 2 mins in normal Stock Android 8.1.0. Configure it slightly (especially the wifi) and on to the next step

Step 4​

Boot again in TWRP mode

fastboot boot $D/twrp-3.5.2_9-0-bullhead.img

Now transfer the Kali NetHunter image from the PC

adb push nethunter-2021.1-bullhead-oreo-kalifs-full.zip /sdcard/nethunter-2021.1-bullhead-oreo-kalifs-full.zip

At the end (~20mins) run Wipe Davik/Cache

Select and run reboot > system

Wait for the ~1min for 1st boot

Enjoy !