Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
Once you have completed thse steps, all of your messages will be deleted from all of your devices connected to the same Apple ID.
Source: https://www.devicemag.com/does-deleting-messages-from-apple-watch-delete-from-iphone/