It takes time but there is only one success. Android9 and 8 failed with boot loop...
FINALLY SUCCESS WITH Android9 on 2023/10/19!

get a good root 2nd time and after. See below for the first time.
Take a minute to backup your data before proceeding. Practical Use: You will interchangablly use two boot images;

1. one for root,
   1. G9500.img: install by "install image" on TWRP as boot.
   2. Magisk.zip (v20.3): install by "install zip" on TWRP.
2. one for de-root or one-seg.
1. boot_cud4_noverify_bless.img
These files are located in /00root/01root/ and /00root/02de-root/.

1. power off,
2. go back to recovery mode by vol-up, bixy and power button, # or $ adb reboot recovery
3. TWRP: install->install img -> choose a boot.img
4. choose boot partition (among boot, recovery, system partitions)
5. and boot up the system.

1. 9500.img with magisk-v20.4 patch; this gives you a good root.
   • modify /system/vendor/etc/fstab.qcom; replace "forceencrypt=footer" with "encryptable=footer".
     this is done from you pc.
     1. mount /system from TWRP on your phone.
     2. $ adb shell from you pc.
     3. # vi /system/vendor/etc/fstab.qcom
2. cud4 official boot.img (One-seg, one-seg, bixy and etc are enabled.)
   • modified by bless hex editor; replace ",verify" with " ".
   Simply use "boot_cud4_no_verify.img.

1. Root Success2:
This one took time!
• Andorid9(CUD4),
• TWRP3.6.2_9-0,
• 9500.img
• magisk-v20.4
Problem: No One-Seg(TV).

2. Root Success1: It was not hard.
   • Andorid7(AQL1),
   • TWRP3.6.2_9-0,
   • no-verity-opt-encrypt-6.0,
   • magisk-v16.4

Reference:

• XDA: SC-02J, Android8(Oreo), magisk-v16.0
  
  • you can download LeoFusionKernel from here, including 9500.img and magisk-v20.4, which enable you to get root.
  • This gives you a hint of veryfy option to /system, which you need to somehow remove it.
  • Also this gives you a tips of forceencrypt to /data, which you need to somehow remove it. (These two things are done by no-verity-opt-encrypt.zip.)

Android9:

• Official Firmware CUD4 boots up.
• It also boots up after TWRP installation.
• It won't any more after "Modification on" at the begininng of TWRP.
• It will again after you install the same version above (for Android7).
  • Andorid9(CUD4),
  • TWRP3.6.2_9-0,
  • no-verity-opt-encrypt-6.0,
  • magisk-v16.4

The Cause of Boot-Loop: fstab.qcom.

1. extract boot.img of CUD4.
2. open the boot.img with bless (hex editor) in linux
3. search and replace ",verify" with " " (7 spaces). (3 replacements)
4. save the modified boot.img as boot-noverity.img
5. intall the new boot.img via twrp.

• https://xdaforums.com/t/oreo-fstab-qcom-in-ramdisk.3711781/

Tips before you start:

1. Use USB2.0 port if oddin complaints Auth Failure during TWRP injection.
2. Remember Root process is:
   • Galaxy s8 variant dependent.
   • Firmware dependent.
   You should know the variant of galaxy s8. (Galaxy S8 is not enough.) You should know your firmware. (Android version is not enough.)

• Docomo Model: SC-02J or SM-G950D, Japan.
  Similar model: SCV36 SM-G950J, au model, Japan.
• CPU: Qualcomm SnapDragon835. (there is another model with Exynos.)
• Android-9: Firmware: SC02JOMU1CUD4 (rooting is firmware dependent.)

• Android Version: (Root is android version dependent.)
  • Android 9: Pie
  • Android 8: Oreo
  • Android 7: Nougut
• Galaxy S8 Variant (Root is s8 variant dependent.)
  • SM-G950F: global model
  • SM-G950D: docomo model, japan. SC-02J.
  • SM-G950J: au model, japan. SCV36.
  • SM-G950U: usa model
• Firmware for SC-02J: https://samfw.com/firmware/SC-02J
  1. SC02JOMU1-CUD4 9.0 20210502 Failed. Boot-Loop.
  2. SC02JOMU1-CSL1 9.0 20191205
  3. SC02JOMU1-CSJ2 9.0 20191016
  4. SC02JOMU1-CSH1 9.0 20190813
  5. SC02JOMU1-CSF4 9.0 20190621
  6. SC02JOMU1-CSD2 9.0 20190404
  7. SC02JOMU1-BSA1 8.0 20190131
  8. SC02JOMU1-BRK1 8.0 20181108
  9. SC02JOMU1-BRJ1 8.0 20181030
  10. SC02JOMU1-BRH1 8.0 20190806
  11. SC02JOMU1-BRF3 8.0 20180627 No Link available.
  12. SC02JOMU1-BRC7 8.0 20180402
  13. SC02JOMU1-AQL1 7.0 20171205 Success! twrp-3.6.2_9-0-dreamqlte.img.tar + no-verity-opt-encrypt-6.0.zip + Magisk-v16.4.zip
      Note: you should use "HOME_CSC" (There are two CSC files) when you write the firmware.
  14. SC02JOMU1-AQJ2 7.0 20171023
  15. SC02JOMU1-AQH2 7.0 20170831
  16. SC02JOMU1-AQH2 7.0 20170809
  17. SC02JOMU1-AQF2 7.0 20170620
  18. SC02JOMU1-AQEI 7.0 20170601
  19. SC02JOMU1-AQDI 7.0 20170428

• SM-G950F: global model
• SM-G950D: docomo model, japan. SC-02J.
  • SC02JOMU1-AQL1 7.0 20171205 Success!
  • SC02JOMU1-BRF3 8.0 20180627
• SM-G950J: au model, japan. SCV36.
• SM-G950U: usa model

1. Youtube: How To Root Samsung S8 G950F All Models Android 9.0 Pie || Tested On Samsung S8 U15 TWRP Root
2. S8+, sc03jomu1AQH2, android 7 in JP
3. Root with magisk without twrp.
4. Root with magisk without twrp (in JP).
5. root galaxy note9 with magisk without twrp (in JP).
6. Root with magisk without twrp (in JP).