While the OTP isn't really needed once A9LH is installed, if the file is lost, than here is a guide to get it back if you really want it.

1- Back everything on your SD Card to a folder on your PC. Then launch Hourglass9 and make a NAND backup. Then rename it to sysNAND-A9LHAX.bin as well as sysNAND-A9LHAX.bin.sha and put it somewhere on your PC.
2- Follow these steps https://3ds.guide/9.2.0-ctrtransfer , then restore your original Health and Safety through Decrypt9. (Should be named "hs.app")
3- Download SafeA9LHInstaller and copy all the files to the root of your SD Card, even replacing arm9loaderhax.bin.
4- When your system boots up, press the button to uninstall Arm9LoaderHax.
5- Then when your system boots up, follow https://3ds.guide/ and get to the point where you downgrade to 2.1. Then, download OTPHelper https://github.com/d0k3/OTPHelper/releases and extract all the files and folders to the root of your SD Card. Now, follow these steps.

1. Go to http://dukesrg.github.io/2xrsa.html?arm11.bin on your 3ds
2. OTPHelper will launch, then select the "Dump otp.bin (0x108)" option
3. Go back to the main menu, then Select the "Dump otp.bin (0x100)" option
4. Press Select on the main menu to eject your SD card
5. Put your SD card in your computer, then copy over otp0x108.bin and otp.bin to a safe folder on your computer.
   
6. Ensure that otp.bin is 256 bytes and otp0x108.bin is 264 bytes
7. Backup otp.bin and otp0x108.bin to multiple locations (such as online file storage)

7- Next, follow these steps.

1. Reinsert your SD card into your 3DS then press B to return to the main menu
2. Go to "NAND Backup & Restore"
3. Restore SysNAND from sysNAND_A9LHAX.bin
4. Cross your fingers
5. Reboot!

8- Now, format your 3DS's SD Card (MAKE SURE THE OTP AND ALL THE NAND BACKUPS ARE ON YOUR PC!!!) and copy over all of the files you backed up in Step 1 on your PC to the root of your SD Card.

And that's it! Also, credit to Plailect for parts of Step 5 and 6.

 

[dAVID_]

Luma CAN boot on 2.1, you just have to delete home menu extdata.