No, there never was a rule. You could hope that people would restrain themselves to only hit you offline, but that's about it.
In war, you gotta expect to be hit all the time, and when gang combat has been changed like it is now, hitting offline players doesnt pay off, only hospitalizing online players does.
This is why people gotta think about who they declare war on. You may be able to beat them in the gang war, but you gotta think about consequences before you declare war.
I'm talking about you as people in a general term.
People have been really nice about the unwritten rule with offline hitting, almost more nice then we could all hope for, and I do hope it stays that way