Re: for manufacturers to open-source old code
I don’t know about Android, but on iOS devices you’ll need a signing key for any update to be accepted by the phone. And that is essential security, to avoid malware to be installed. Even older original iOS updated are not signed anymore, since hackers might have exploits against an older version which has since been fixed, and you don’t want them to be able to remove these fixes.
Now if you had the source code for the last iOS version running on an iPhone 5 (that would be the newest iPhone without security updates), Apple isn’t going to hand you over signing keys. That would be madness. And how many people would be interested in a new version?