Statistically, it's a clear enough statement, but to be fair, we should treat it as though it isn't.
It isn't fair that people who ask questions about industry-leading product Dungeons & Dragons Fifth Edition can get useful answers to their questions without even having to name their system, while people who play anything else, even if they correctly name and tag their system, get non-useful answers from people who assume any question they recognize D&D 5E terms in must be about D&D 5E.
People playing other systems can encounter scenarios where they want to put armor on in combat, or mitigate critical hits, or be a druid and turn into a blink dog.
And yes, the answers that assume those questions are about D&D 5E get downvoted with a quickness, but that's also kind of a problem, in that often those answers will get deleted and low-rep users viewing the question don't see deleted answers. (Or answers hidden because of very low score? Is that a thing too?) So it's just an empty question that they think they know the answer to, and then the original question gets another unhelpful answer and the respondent gets another tranche of downvotes.
It seems like a waste of time and opportunity to wait for some kind of explicit statement of "I am playing industry-leading product Dungeons & Dragons Fifth Edition" from the original poster, even if that's just adding it in as a tag to not break the conversational tone of the question, before a question is opened for answers. But I'm still in favor of doing it to establish the contrapositive; that is, if there's a question that's open for answers, and neither the title nor question body nor tags mention industry-leading product Dungeons & Dragons Fifth Edition, people can be sure it's not a question about industry-leading product Dungeons & Dragons Fifth Edition even if they don't know the system that it actually is about.
Because it also isn't fair to present people without much experience outside industry-leading product Dungeons & Dragons Fifth Edition with a front page full of questions where they can tell that some are definitely about D&D 5E, and some just seem like they might be about D&D 5E and other people have assumed they are and answered, and some seem like they're about D&D 5E but nobody's answered, and the way they're supposed to tell which ones are okay to assume and which ones aren't is... a volatile list of acceptable substitutions on the meta?
I can't read somebody else's mind who writes an answer, any more than I can read somebody else's mind who writes a question, but of the five people to swing and miss at those other system questions, one has continued giving answers. Just a couple, and they had a gold badge already.
Part of this unfairness is that we don't have a way to clearly specify the game system in a question. A proper noun could be any number of things in the big tent of industry-leading product Dungeons & Dragons Fifth Edition; a setting, a structured play environment, an unfamiliar game element. It's highly probable that a system tag for that proper noun exists, but does every system tag mention it's for a game system? There's no way otherwise to tell system tags from non-system tags.
The judgment of whether or not someone has specified a game system is left up to people who volunteer to flag and review. It's the best we can do for now, and I hope they have a good degree of awareness of what game systems are out there, but when they're making that decision I'd like to leave them a few clear guidelines rather than an arbitrarily-sized list of exceptions.
Because, lastly, it's not fair to the future. Industry-leading product Dungeons & Dragons Fifth Edition might not be industry-leading forever. As time stretches on, all human projects will dissolve and fail, and I'd rather maintain a clear identification policy than have to keep revisiting acceptable alternates for popular games as time goes on.
SELECT * FROM questions WHERE (question_text CONTAINS '5e' OR question_text CONTAINS '5th edition') AND tags !CONTAINS 'dnd-5e' AND tag_edit_history CONTAINS 'dnd-5e'(yes I know that's not even good SQL, my point is "was anything that said '5e' in the question ever tagged dnd-5e then later untagged?") \$\endgroup\$