Comment 4 for bug 964882

Not being able to set autoconf=0 when using static is certainly a situation where you want ifup to fail.
When you set a static IP you usually don't want to get SLAAC to happen and set some extra IPs on your interface.

Marking the ifupdown task as invalid, systems are expecting to have writable /proc/sys and the network namespaces work perfectly fine for anything in /proc/sys/net so no need to restrict that by making it read-only.