Setup: RasPi 4b w/ 8GB RAM booting of USB3 SSD

What worked: booting with the old 5.4.83:
  • both with screen attached, or headless
  • powered by USB powersupply, or by computer

What works: booting with the new 5.10.103:
  • both with screen attached, or headless
  • only powered by computer

It was difficult to track this down, because it was just due to the setup I have, that hooking the RasPi up to the screen meant I powered it for convenience from the computer. So at first it seemed the booting was dependent on a screen being attached, so I chased the wrong leads.

In reality, it's the power source, but that likely has a different reason:

I have USB-C networking enabled, so I can travel with the RasPi and hook up an iPad Pro with MagicKeyboard and access the RasPi with a fast, secure connection by xrdp via USB-C networking, and power the RasPi over the same cable (iPad Pro can easily be powered in turn by the USB-C port on the MagicKeyboard, but I digress...)

So what seems to have happened: with the change in kernel from 5.4.83 to 5.10.103, and the corresponding other updates, the USB-C-OTG networking was messed up. When enabled, the system only seems to boot when connected to a computer or some sort of USB host.
But as if that weren't enough: when that's the case, ifconfig reports the interface being up:

usb0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500        inet  netmask  broadcast
        ether 8a:d5:e4:fe:32:ef  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
But the host (in my case a Mac) sees the device as down.

So there have been some significant changes/bugs introduced with the latest updates, in that USB-C-OTG networking seems to be broken in significant ways...

There's one other option: that the way to enable/setup USB-C-OTG networking as significantly changed, but I'm not aware of any relevant changes in that regards.