Are numbers greater than 20 legal hints in Codenames? I.e., "911, 3" for "plane", "event", and "skyscraper"
Codenames enjoys a variety of flexible rules and often it's up to the opposing spymaster to allow cases like this. 9/11 could definitely fit in either of the following 2 flexible rules, but since they are flexible rules, it's best left to the group to decide if it's allowed or not.
While technically wildly different, 9/11 is not just a date, but it's the common name used for an event. Just like Super Bowl are two words, most groups would allow it as a compound word.

Compound Words
English has three ways to write a compound word. Greenhouse is one
word. Pack rat is two words. Mother-in-law is hyphenated. Technically,
only greenhouse can be a one-word clue. You can decide to allow any
compound words. However, in no case should a player be allowed to
invent compound words. Lunar squid is not a valid clue for MOON and
OCTOPUS.
Acronyms and Abbreviations
Technically, CIA is not one word. But it is a great clue. You can
decide to allow common abbreviations like UK, lol, and PhD. And words
like laser, radar, and sonar are always allowed, even though they
originated as acronyms.

Remember, don't be too strict. If the opposing spymaster allows it, the clue is valid. If you aren't sure, ask your opponent. (Quietly, so the others can't hear.)