The knowledge hasn't been lost, but the patches to make it work are large and invasive and much of the code needs to be rewritten from scratch as GBM/V4L2 works completely differently. The work is not impossible to do, but one goal of the GBM/V4L2 work is to upstream everything so that all distros have good media performance on Pi hardware (with a focus on RPi4) not just LE/OSMC who were mad enough to entertain a 50,000 line patchset in the past. The nature of the changes needed for optimised HEVC means they will be hard cum impossible to upstream, and while the percentage of LE users that might want the patches is high, most other distros with an actual patching policy refused the existing patches, so the total percentage of Pi users (all distros) that want the patches is low - and hence the Pi Foundations motivation for reinventing this capability is pretty low. Forcing updates is not their motivation at all, but a few users updating to RPi4 to gain more features including hardware HEVC decode doesn't hurt either.