Bonjour ! Oui, je confirme que les français de France utilisent aussi l'expression "de bon matin" ou "de bonne heure" couramment !! Cordialement.

Is it formal or informal? To me it's completely correct. Et aussi "tôt le matin" "à l'aube"...

In the French of south Louisiana, “de bon matin” means “early” or “in the morning” (Faut qu’alle soye là-bas de bon matin – She has to be there early.) This is, of course, the Cajun French of Louisiana, not to be confused with Parisian French.

They still used it

In Québec, people do use "bon matin" to say "good morning", but it is technically not correct, and "bonjour" should be used as a greeting...