The device I use was developed very specifically for class D audio, not for power supplies, low voltage points of load or RF circuits.
Yes, others use GaN devices but not the ones he worked on with the supplier and he bought out all the inventory. I don't know where you got the notion that the circuit is obsolete unless it was meant to demean.
He's devising a new one to deal with the increased power of his next amp. Going back down in power doesn't appear to involve modifications. That, and although other makes use some form of GaN device, they're not intended for audio, like his are.
And, yes, I took some poetic license in saying that it was for radar as I was lazy to not list all the other applications. You're quite the pill, aren't you?
All the best,
Nonoise