User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange
User
HKCU\software\policies\microsoft\office\16.0\outlook\emsp
This policy setting controls the creation of ANSI OST files. If you enable or do not configure this policy setting, new ANSI OST files cannot be created. If you disable this policy setting, all new OST files for an Outlook profile are created in ANSI format. Profiles with multiple Exchange accounts will always create Unicode OST files, regardless of this policy setting.