There is no standard, not even de facto. If you want to use a barrel connector in your product, you just write in the manual that only the supplied adapter is allowed and using anything else voids warranty, and then you let users either be careful choosing the right voltage and polarity, or kill your product and (hopefully) learn from that.
Luckily, with USB power everywhere, DC plugs are rarer today and as such, users are less likely to pull any random power supply and try to use it with your product instead of your supplied adapter.