User Configuration > Administrative Templates > Microsoft Word 2016 > Word Options > Proofing > AutoFormat as you type > Apply as you type
User
HKCU\software\policies\microsoft\office\16.0\word\options\assist
This policy setting allows you to set the "Closing style to letter closings" option found in Word Options | Proofing | AutoCorrect Options... | AutoFormat As You Type. If you enable this policy setting, "Closing style to letter closings" will be set. If you disable or do not configure this policy setting, "Closing style to letter closings" will not be set.