'5e' unambiguously refers to Dungeons & Dragons (2014), as long as it's clear that's intended as a name
There are many systems that have a 5th edition. Sometimes new querents asking a question badly phrase their question like this:
Hi i was wondering if trolls can cast spells thx. [Edit]5th edition.
That's not clear. 5th edition may be being used to describe what edition of the system is being played, rather than naming the system itself. For example, it's possible the above question is being asked about Shadowrun rather than D&D.
Other times, new querents might ask a question like this:
We just started playing 5e and I wanted to have the party fight a troll that casts spells. Can trolls do that?
This question is still not great, but it's no longer unclear what game they are playing. 5e here is used as a name, and right now there is only one game called '5e'.
D&D-centrism is bad, and 5e centrism is worse! But this isn't the way to solve it
Our site's 5e centrism is out-of-hand, and a problem. For example, we recently had a new user get repeatedly harassed for asking properly tagged AD&D questions because somebody decided that frequent tag misuse is a good enough excuse to take questions in bad faith (for the record, it's not and that's offensive). Coming up with good solutions to our 5e-centrism problem would be a good thing to do. This is not that.
Passive-aggressively singling out 5e players for obtuse facetious BS as a barrier to site entry is not cool. It's Not Nice, which is a serious problem that should bear consideration, and it's not actually productive because we rely on common-sense judgements for pretty much the rest of our site's functionality so it basically reverse-trains new users into thinking about what our site policy can and can't do the exact wrong way. It's just mean-spirited, ineffective moderation and we should stop doing it.
That's not to say our 'don't guess' policy needs to go away. We just need to do a better job of treating every system's community as justly as we can when adjudicating this policy-- we don't treat any other system the way we do 5e with this. For example, one of my favorite systems is Polaris. I've never had a question closed as unclear which system I was playing when asking about that, even though there is this other system which is literally named the same thing and which does not yet have a separate tag. If we treated my Polaris questions like we do 5e questions they would all have been closed until I specified not only that I was playing Polaris but also that game's ISBN. That's absurd. No expert on either system would ever confuse any of my questions for a question about the other system. Not 'probably wouldn't', wouldn't, hard stop.
If a ask a question that's obviously about Shadowrun and don't include an edition or system tag I don't get a patronizing formula-comment asking what system I'm playing and what edition as if that question made sense without already knowing I'm playing one of the very few systems where 'edition' is the right term and matters, I get the question closed as unclear and one person asking "Is this Shadowrun? Also what edition-- the Matrix rules vary a lot" or, if it's clear what edition, "Is this SR4?". That's what I should get on 5e questions; a closed question and an honest comment that tells me I need to confirm that I'm playing D&D 5e. If it's actually unclear, it's totally fine to ask. But pretending like you don't know when you totally do know is just being a jerk.
It's worth noting that the downsides of our bad behavior here are particularly intense for 5e. Wizards made the very-bad decision when releasing the game to hide the edition as much as possible in order to accomplish business objectives by misleading people. This has, predictably, lead to lots of new players who have no idea what edition they are playing and no idea how to find out because that is extremely non-trivial for the newest edition especially if an online community of experts is acting like it should be obvious.
In any case, I think the best thing we can do here is to treat 5e like any other system for this policy as much as possible. We should avoid guessing system tags in edits, but not in comments. We should accept commonly-used colloquialisms/abbreviations like 'SR5' or 'D&D 3.5' or '5e' or 'LotFP' or 'Dungeons & Dragons 5th Edition' as names as long as they are clear and unambiguous and widely accepted. We should post honest comments that simultaneously respect the intelligence we assume of our querents because we give people the benefit of the doubt while also acknowledging their inexperience with the site.