We’ve always had this kind of weird limitation where if you’ve never played WoW we let you play as much as you want, albeit in a very restricted Starter Edition account.
However, if you have ever had a subscription--but don’t currently have one--we don’t even let you log in.
In 6.1 we’ll effectively revert lapsed accounts into a Starter Edition “mode”, with all of the same restrictions. The one difference being that your sub-level 20 characters will be able to join a guild if any of your other characters are still in that guild.