Officially confirmed to be impossible.

Answer from GitHub support:

Thanks for reaching out to GitHub Support about using GitHub Pages

 

We set the following Cache-Control header for all GitHub Pages content:

 

Cache-Control: max-age=600

 

This header is the same for all assets on all sites on our Pages service, and we don't currently provide a way to alter the value.

 

I hope that this answers your questions.

Officially confirmed to be impossible.

Answer from GitHub support:

Thanks for reaching out to GitHub Support about using GitHub Pages

We set the following Cache-Control header for all GitHub Pages content:

Cache-Control: max-age=600

This header is the same for all assets on all sites on our Pages service, and we don't currently provide a way to alter the value.

I hope that this answers your questions.

Officially confirmed to be impossible.

Answer from Github support:

Thanks for reaching out to GitHub Support about using GitHub Pages

We set the following Cache-Control header for all GitHub Pages content:

Cache-Control: max-age=600

This header is the same for all assets on all sites on our Pages service, and we don't currently provide a way to alter the value.

I hope that this answers your questions.

Officially confirmed to be impossible.

Answer from GitHub support:

Thanks for reaching out to GitHub Support about using GitHub Pages

We set the following Cache-Control header for all GitHub Pages content:

Cache-Control: max-age=600

This header is the same for all assets on all sites on our Pages service, and we don't currently provide a way to alter the value.

I hope that this answers your questions.