Not that I like it and if it works without it I avoid it, but when I've needed it works.
Let's hope there is no basilisk.
Another one I tried is when I had it helping me with some Python code. I told it to never leave trailing whitespace and prefer single quotes to doubles. It forgot that after like one or two prompts. And after reminding it, it forgot again.
I don’t know much about the internals but it seems to me that it could be useful to be able to give certain instructions more priority than others in some way.
I've had much better experiences with rephrasing things in the affirmative.
There is no magic prompting sauce and affirmative prompting is not a panacea.
That's not how YOU work, so it makes no sense, you're like "but when I said NOT, a huge red flag popped in my brain with a red cross on it, why the LLM still does it". Because, it has no concept of anything.
A lot of these issues must be baked in deep with models like Claude. It's almost impossible to get rid of them with rules/custom prompts alone.