Are the two words "synonym 2"/"similar 2" and "opposites 2"/"antonyms 2" valid clues in Codenames? E.g. "opposites 2" for "quiet" and "shout"
From the rules:-

DON'T BE TOO STRICT

and

If the opposing spymaster allows it, the clue is valid.

and

Sometimes you have to make judgment calls about what is valid and what
is not. Different groups may prefer to play the game differently.

For any question about if a clue is a valid all the above applies.  Unless there is a rule which prohibits a clue its more than likely ok.
I've seen clues given for the meaning of groups of words.  A team had just &quot;ICE&quot; and &quot;QUEEN&quot; left to guess and gave the clue &quot;NARNIA 2&quot; to hint at those which for out group was OK.   As long as you don't say things like &quot;this is for a group&quot;, or &quot;this is a stretch&quot; then its fine.