How to rollback to unlock bootloader?

Hello, I have GM1917 on OOS12. Trying to install LineageOS to extend life of device. I'm stuck unable to unlock the bootloader and the recommendation is to rollback to OOS11.

I found the post on OnePlus community forums but it seems all package links are dead. I would be comfortable using MSM but also couldn't find the correct files for the device.

Anyone have ideas?

Thanks!

edit: MSM links below, use GM21AA package to go back to 11.