Earlier quoted context omitted.
Whilst the support might be "outstanding" - the discussion is what happens once devices are no longer supported.
You can't release all the documentation just because the entire phone isn't supported. Many of the components come from other suppliers and aren't obsolete, and you can't just reveal all your suppliers' IP.
It's not a security problem, since they don't support it anylonger anyways!
They could even make it so, that iOS itself refuses to boot if the device is unlocked. That way you can't accidentally have an iOS running that's compromised in some way.
But you can still boot Linux or Android or whatever you want to do to it.