Comment by snickerbockers

the usual license header has something along the lines of "either version [23], or at your discretion, any subsequent version", which clearly explains that there are specific versions with distinct rules. Many people opt not to include this clause because they (understandably) don't want to automatically agree to a contract that hasn't even been written yet. However if they fail to make the version clear that's on them.

Anyways I don't think this defense would ever fly in court. As soon as the plaintiff's lawyers produce evidence that you are aware of GPLv3 (such as pointing out that you have GPLv3 software on your PC or phone) the judge is going to see that you're trying to game the system on technicality and sanction you. Judges really don't like this sly loophole BS where it's extremely obvious that you're feigning ignorance for the sake of constructing an alternate reality where you hypothetically never knew there was a GPLv3.