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.
Press and hold the text you want to unsend until a menu pops up.
Source: https://www.slashgear.com/1315419/how-to-unsend-messages-iphone-guide/