- crossword clues
- cryptic crossword clues
- word ladders
- Scrabble moves
- substitution ciphers
- solving a puzzle
- composing a clue
I see no reason why these should be off-topic. Even though there's already an English site, puzzles about the English language would be more relevant here instead. (Same argument for math puzzles.)
However, if the question is not specific to puzzles, ore could be rephrased in a more general way (such as "What is the most common letter in the English alphabet" - bad example but there are definitely more), then it would be better off on English Language & Usage.
No; we explicitly don't want riddles because they could have multiple opinion-based answers, making them a "guessing game."