It's always been an rule, just one not written down, just implied.

This is why Jiro posted the rule because a lot of people were getting confused just like you have here. We have seperated the Summon and Pet sections so they are different plus we have added a sticky about it. Summons are animals that you fight with in battle and have jutsu's thye also require a summoning contract. Pets are companions which you can RP with socially etc and travel with you.