There is already an issue on isaacs/github¹ about this, but I’m not sure whether that got much attention. I thought this might be a better place to ask: Can we expect HSTS support to be available for GitHub pages at some point? The current lack of HSTS makes GitHub somewhat less secure than alternatives and it seems like a rather simple feature to implement. Thanks for considering!
¹ with 120 thumbs up reactions at the time of writing