I've read a few of these over the years, and none of them seem to be useful. I have three sentences in my custom instructions, and those are basically all useless, too.

Even my second one, "Avoid decorative or section-header comments. Never use `----` or `====` as comment separators. Comments should explain only non-obvious behavior, rationale, constraints, or implementation details." seems to be ignored by models regularly, so I don't see the point.

But this is in my private harness. Perhaps other harnesses have better instruction following. My custom instructions are prepended to my first user message, not set as a system message.