Thanks @JammyDodger! Your answer saves tons of my research time… I didn’t even know there’s such a settings even search through all emails related setting and posts in this forums and I even debugged into code level…
What’s the default value of this settings, it’s “no” I assume? My previous setting is “non-stuff” I don’t know when it’s set? Is it when “migration” as mentioned in this post Send only system emails when disable email setting is on - #5 by Stephen? I did do a data restore operation to the site before signing up new users account.