I really don't see how that is an option, as if appending to the system prompt was ever enough to override previous instructions. Their example isn't very confidence inspiring either:

"The user switched the workspace to read-only mode. Do not write files until told otherwise."

Great! Now we just have to trust that the model never misinterprets any of the system prompt, which has always been so reliable before. Instead of your meticulously crafted prompt, it will now be some junk like this:

"The workspace is in write mode. The user switched the workspace to read-only mode. Do not write files until told otherwise. The workspace is now in write mode again. Wait, back to read-only!"

And who knows how this integrates with their context summarisation that we will be FORCED to use. How does it summarize multiple user + assistant/thinking blocks without messing up the system "appends"? If all it did was append the mid-convo system messages right under the original system prompt then they'd be ripe for all the same distillation "vulnerabilities" as before. I guess we'll never know!