This only concerns the kernel which is rather unimportant when it comes to Android updates. You can keep using an ancient kernel for an insanely long time but upgrade the Android userspace. The vast majority of LineageOS devices use the original kernel they released with (+bug fixes, usually).
Only when Android has a hard requirement on a new kernel feature do you need to actually upgrade it. This is usually end of the line for a device in custom ROMs because it is infeasible to do in most cases.
Take the Oneplus One (bacon) for example. It was released oven 9 years ago with kernel 3.4 and only lost LineageOS support with Android 12 because that requires eBPF for firewalling apps which is a relatively recent addition to the kernel.
The shims for the HAL you mentioned are in userspace. The original BLOBs they shim use the original OEM kernel interfaces in order to do their magic. It’s just that Android might require newer/different interfaces from the HAL BLOBs; that’s what the shims are for.