My late grandpa always used to tell me that "a boy" is "der Knabe" in German. Is this word obsolete now?
Depends on where you're from.
https://www.duden.de/rechtschreibung/Knabe says that it's veraltend, sonst gehoben, Amtssprache, schweizerisch, österreichisch "obsolescent, otherwise 'posh', bureaucratic language, Swiss, Austrian".
Where I live (Hamburg, northern Germany) that's certainly true: Knabe sounds extremely old-fashioned to me.
But it may well be alive and well in Austria, for example.
When I was still a full contributor, our team had the policy not to accept Knabe (or Bursche or Bube or other regional words for "boy") since this course focussed on German from Germany without regionalisms as far as possible -- so words that were only used in Switzerland or Austria or only used in parts of Germany were generally not accepted.
I'm not sure what the policy of the current group of course contributors is; they may be more lenient on this point. (But if so, it would take a while to go through all the thousands of existing sentences to adapt them to any change in policy.)
It was a "type what you hear" question and there was no voice. That ended an all correct answers streak, the best I had done so far. On another "no voice" question in a previous lesson I had typed in "nichts" for the answer to what was heard, but it was marked wrong.