I'm in favor. I don't really care whether we do things the same way as in Reddit but sometimes I start typing a reply to a thread without noticing which community it's in. Having ELI5 in the title makes it immediately clear what kind of responses are wanted.
That said, if the rule is implemented, I don't think it's necessary to start enforcing it strictly right away. It's not that big of a deal if someone forgets the rule. That way it wouldn't create any extra work for the mods.