I am curious, who has discredited this rule? I wouldn't say it is proper to end with a prepostition. It is accepted because people do it and many English-speaking people do not know the English rules of grammar. I do think with the number of non-Engish speakers using Duolingo, and the fact that it is an educational site, it behoves them to show an answer that is grammarically correct. (Mind you that I have probably made grammarical and spelling mistakes here and in no way claim to be perfect. I just enjoy the dialogue and no offense meant).
I was first taught the rule as a child from Strunk & White, beloved of my mother who was an English teacher. That book has other silliness in it now discredited as well. Thanks to google here's a site on the subject: http://grammar.about.com/od/grammarfaq/f/terminalprepositionmyth.htm It seems the origin of the idea lay in rules of Latin grammar, not entirely suitable to English and so an awkward fit. One quotation among many: "It was John Dryden, the 17th-century poet and dramatist, who first promulgated the doctrine that a preposition may not be used at the end of a sentence. Grammarians in the 18th century refined the doctrine, and the rule has since become one of the most venerated maxims of schoolroom grammar. But sentences ending with prepositions can be found in the works of most of the great writers since the Renaissance. In fact, English syntax not only allows but sometimes even requires final placement of the preposition. (The American Heritage Book of English Usage, Houghton Mifflin, 1996)"
Winston Churchill. http://public.wsu.edu/~brians/errors/churchill.html
In all seriousness, most style guides take the line that it's usually an indicator of poor style and thus to be avoided, but preferable to an unnatural or excessively long-winded sentence.