[Samsung Galaxy Tab A series Questions & Answers] Winston Churchill:
I will write out what I just got to work with the latest update of Android Pie for the Samsung Galaxy Tab A T380 running T380DXU3CSL2, security patch January 2020. I have tried all of the methods on the main TWRP/Root thread (which is now closed) and only had success with one.

To clarify, yes - using the patched boot.img from ashyx that was made from T380DXU3CSI5 WILL work with T380DXU3CSL2 (latest as of time of writing) just fine. Make sure to flash Stock first and OEM Unlock.

Ashyx does give some files still needed in his/her first post on the main TWRP thread, but the boot image and the DM-verity are buried within a 66 page thread. Out of respect for ashyx's wishes I will not direct link to files. Get TWRP from the first post here. The patched boot image and DM verity no encrypt get from the guide here by user zfk110. Pg. 65 of the TWRP thread. The guide itself did not work for me though. Just grab the files.

So, files you will need:

>twrp_3.2.3-1_sm-t380_oo_4119.tar (TWRP)

>T380DXU3CSI5_patched_boot_111119.tar (boot.img inside the .tar)

>Latest Magisk version here

>Tab_A_2017_Pie_forced+encryption_disabler-1.zip (no-verity-no-encrypt)

Not only have I found RMM Bypass unnecessary, but in my many trials I think maybe it was causing an issue for me. Perhaps someone much smarter than I can explain, but there is no setting in build.prop to need it - I did check.

TWRP/Root:

>Flash TWRP with Odin with "Auto Reboot" setting turned off: (file: twrp_3.2.3-1_sm-t380_oo_4119.tar in AP slot)

>Boot straight to TWRP (home + vol down + power to get out of download mode then as soon as the screen flashes swap to home + vol up + power),

>In TWRP main menu press "Wipe" and the "Format Data", type "yes" to proceed.

>Reboot to TWRP by going back to main menu of TWRP, select "Reboot" then "Recovery"

>Install boot.img to your boot partition from your external SD card in TWRP (there is a YT video how to do this within TWRP if you need help, just Google it. The file you need is: T380DXU3CSI5_patched_boot_111119.tar then extract boot.img from that for TWRP (use ZArchiver or a program that will unzip .tar). It MUST be in .img to install it with the TWRP "Image" button. TWRP won't even read that the .tar is there, and I don't recommend Odin for this)

>Install Magisk from your external SD card (I used the latest, 20.4 just fine)

>Install the DM-Verity Forced Encryption Patch from your external SD card (file: Tab_A_2017_Pie_forced+encryption_disabler-1.zip)

>Wipe Cache and Dalvik

>Reboot to System

Notes:

Boot image must be first before flashing the others. I tried it after Magisk et.al as with a number of people's directions and several other configurations in addition both in TWRP and with Odin (and the other boot image as well on the thread: t380_boot_pp.img - no luck on XSA for me at least) and it caused a bootloop every time. I don't know why. The smart guys are on the TWRP thread but it's closed (and confusing). I just try things.

To the other OP question here - use the later version of the DM-verity patch - the original one (no-verity-no-encrypt_ashyx) you are referring to is a different size and structure (I have it archived) so it is probably necessary to use the Tab_A_2017_Pie_forced+encryption_disabler-1.zip. It's on pg.65 on zfk110's guide that I linked above (though again, the guide itself did not work for me).

Edit: I know someone could find this method out from the big thread but I know what it is like to feel newby and get confused and want to give up. And the number of different methods and files on that dang thread was a bit maddening honestly and frequently in direct conflict with each other.