This is what Copilot told me, not sure if it is correct:

The Rushstack guidance (followed by many TS codebases) prefers undefined for “no value” in modern TypeScript/JS, and reserves null only for:

• Interop with legacy APIs that already return or accept null (e.g., some DOM or 3rd‑party libraries).
• Data formats like JSON where null is part of the schema.

This avoids the complexity of having to consider both null and undefined everywhere.