Earlier, I've asked a question about whether poetry elements/sound devices like alliteration, assonance, consonance, and rhyme (Is it legal to use poetry elements/sound devices as hints like "alliteration 3", "assonance 3", and "rhyme 3" in Codenames?), and the consensus was that it was illegal because it didn't refer to the meaning of the words but just the way they were spelled. I'm curious about synonyms/antonyms because they refer to the meanings of words, but only the meaning of cards on the board compared to other cards on the board as opposed to the clue itself (I know that sounds complicated, but please read my examples below for an explanation.)
An example of "synonyms 2" would be for "ice" and "water" because it might be hard to come up with a clue that doesn't have water in it--closest I can think of is "cup 2", "moisture 2", "refrigerator 2" or "cold 2" but can hit unwanted cards.
An examples of "antonyms 2" would be "quiet" and "shout". I think "volume 2" could work perhaps there are other cards on the board that are related to volume that you don't want to hit, like "music".
The rules state:
Your clue must be about the meaning of the words. You can't use your clue to talk about the letters in a word or its position on the table. Gland is not a valid clue for ENGLAND. You can't tie BUG, BED, and BOW together with a clue like b: 3 nor with a clue like three: 3. However …
I am thinking it is valid because synonyms are antonyms are about the meaning of the words. However, I'm a bit sketchy about it because it's not the meaning of say "ice" that is related to the clue "synonyms 2", but it is that a card on the board "ice" that relates to another card on the board "water". It doesn't say you can't link cards on the board using other cards on the board, but I would imply that when it says "Your clue must be about the meaning of the words", it could suggest that the meaning of the word must be related to your clue, not the other card. "ice" is certainly not related to "synonym".