>what the BD acronym in “BD rate” stands for,

Bjontegaard Delta-Rate (BD Rate) metric, proposed in 2001 by Gisle Bjontegaard, is a method for calculating the average difference between two rate-distortion (RD) curves.

It is extremely common in codec comparison, along with terms like PSNR, SSIM and VMAF ( which is newer and developed by Netflix so it tends to get explained a bit more )

>’m certainly curious if this is aimed at becoming the new default format for Apple devices.

I certainly hope not. Not unless it is deterministic and much much higher quality.