I’ve always been told no backpacks or small bags in any of the D1 cabins. This includes the 767. I believe on the 757 the placard says 10lbs max, but was told not to leave my bag there.

Most of the time the 757 is boarded from L2, meaning coach passengers won’t walk past your seats. There is plenty of overhead space in the D1 cabin for each seat. In hundreds of flight segments with DL I’ve never had an issue with theft and I often travel with fragile equipment.