GitHub has introduced Content Exclusion for Business and Enterprise users that allow the org owners to setup file exclusions such that those files are ignored by copilot.

https://docs.github.com/en/copilot/managing-copilot/configuring-and-auditing-content-exclusion/excluding-content-from-github-copilot

As per them:

• Code completion will not be available in the affected files.
• The content in affected files will not inform code completion suggestions in other files.
• The content in affected files will not inform GitHub Copilot Chat's responses.

Do note that if you ask Copilot Chat about it, it will simply deny sending over any code at all, but if you were to check the outgoing network requests, you can see entire contents of the current file being sent along with the prompt.