a9lh can't be removed, but it requires an existing arm9 exploit to install it (which is why we downgrade to 9.2). They've already patched all non hardmod/fieldrunners downgrading on 11.0+ (forever), and it's only a matter of time before the patch hardmod downgrading.
Bootrom exploit wouldn't need downgrading (could be used on all FWs), but depending on the nature of the exploit might require specialized hardware.
This is only speculation; we don't know if there's an exploit, and if there was what it would require.