Is it considered a divorce if the husband says to his wife online: "You will be divorced today," while claiming that he was joking and did not intend divorce and did not explicitly utter it?

This is not considered a divorce because it is merely a promise of it, and it has not been uttered. Writing a divorce is considered an ambiguous expression (kinayah) that requires intention. If he did not intend it, then it does not take place. However, one must be cautious about toying with the bond of marriage and exposing it to destruction.