It looks like it might not necessarily forbid using a custom ROM, but it must be based on the minimum required version, if the e-fuse is blown:

Quote from: xdaforums.com/t/critical-warning-coloros-16-0-3-501-updates-permanent-anti-rollback-arb-fuse-blown-do-not-downgrade.4775930/#post-90453464Many users are asking: "If I am fused, can I still flash Custom ROMs?"

CURRENT ANSWER: NO. IT WILL BRICK YOUR DEVICE.

Why?
1. It is too new: The "Fuse-Blowing" update (ColorOS 16.0.3.501+) is brand new. OxygenOS hasn't even widely pushed it yet.
2. Developer Lag: Almost ALL current Custom ROMs for OnePlus 13 / Ace 5 were built BEFORE this fuse policy was known. They are adapted for the "Old" (Unfused) firmware environment.
3. The Trigger: If your phone is fused, and you flash a current Custom ROM, the system will trigger the Anti-Rollback (ARB) mechanism due to firmware/signature mismatches.
4. The Result: HARD BRICK immediately.

Advice:
Until Custom ROM developers explicitly announce "Support for Fused Devices / New Firmware Base", DO NOT FLASH ANYTHING if you have already updated to the .501+ version. You are stuck on Stock for now.

Oh, wow.. Didn't even know this was a thing.

I wonder, if only local CN devices / imports are affected or global ones too. If all, then my main reason to get a OP phone would be gone.

So much for OnePlus. Guess I'm stuck with Pixel phones...

This should be illegal, full stop.

Current own a cheap OnePlus but that'll be my last.