Before you get angry with the question, I know that the the rules say "You can't use your clue to talk about the letters in a word or its position on the table".
I was just curious on a move I made if it was illegal. There were only 4 words that starts with H. The tiles were all our words, so I gave the letter H as a clue. I thought this was a once in a blue moon opportunity. What was I did just plain illegal?
It seems you already know the answer and it is illegal because you are not allowed to talk about the letters in a word. The rules are very clear that the clues you give have to be about the meaning of the word and what you are asking about would be in violation of them.
https://czechgames.com/files/rules/codenames-rules-en.pdf
Your clue must be about the meaning of the words.
Letters and numbers are valid clues, as long as they refer to meanings.
The number you say after your clue can't be used as a clue.
You can't say any form of a visible word on the table.
You can't say part of a compound word on the table.
Same-sounding words with different meanings and different spellings are considered different words
Words that are spelled the same are considered the same
