Interesting. Thank you, I'll check it out.
https://forum.xda-developers.com/t/guide-re-locking-the-boot...
None of the operating systems that are designed to be used with a relocked bootloader (e.g. CalyxOS, GrapheneOS, RattlesnakeOS) support OnePlus phones at the moment, so you'll need to build LineageOS, AOSP, or some other Android variant manually to use it on a OnePlus phone with a relocked bootloader.
There might be more device models with this kind of support. Search for "avb_custom_key" with the model name to see whether anyone has commented on this.