"You did not know about it?"
Translation:Vy jste o tom nevěděli?
Explained below in a comment by VladaFu.
"o tom" can either be first (= topic, stressed) or second (= unstressed). Placing it last makes it the focus and it's unnatural. It works completely fine if you actually specify what "that" is, e.g. "Vy jsme nevěděli o tom skandálu?"
(In "Nevěděli jste o tom." it's still in the second position (at the end of it), which only happens to also be the last position.)
To je stejne jak v cestine, ptame se, jestli tomu tak opravdu bylo. https://www.helpforenglish.cz/article/2009010202-otazky-ano-ne-s-oznamovacim-slovosledem
That is really strange. You are wondering that didn't know specifically ABOUT THAT as opposed to the many other things they did know about. Even if you wanted exactly this, one would normally choose some other words and not "o tom". Most likely one would explicitly name the soecific thing.
Normally the main point of the sentence is the not knowing and therefore the verb goes last.
Vy jste nevěděli o tom úspěchu? You didn't know about that succes?
Vy jste nevěděli o tom, že ... ? You didn't know (about the fact) that ... ?
These two are both fine, but just "o tom" is not natural.