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.
Access to new features is not available, and app developers may no longer support updates to their applications.
But AFAIK, Apple apps (maps, music, phone, iMessage etc) on iPhones no longer receiving iOS updates continue to work.
You won't get updates to the trusted root CAs, which means you won't be able to visit sites with certificates signed by CAs created or renewed after support is dropped. And your browser will continue trusting CAs that have had their trust revoked.
And as web standards evolve there will be websites that use features and APIs that your browser doesn't support and may break in subtle, or not so subtle ways. And there is no way for you to install a more up to date browser.
And then of course, you won't fixes for any new security vulnerabilities that are found.
So yeah, it's not as bad as getting bricked, but it as also worse than continuing to work as it always has, but with no new features.
So, at best the device can just be used with the latest version of the software Apple allows until it's a security nightmare and better off no longer used.
Instead, if Apple gave people the ability to load something (prob a Linux) onto those old devices, then those old devices could be used usefully for quite a few more years.
Did they gave instructions on how to unblock bootloaders. released the source code and drivers under an open source license?
