Can we hide default address lists in Outlook Address Book and show only custom ones?

Hi Como, the usual way to do this is with Address Book Policies, not by simply hiding the GAL in Outlook. You create the custom address lists, custom GAL, room list, and offline address book, then assign the ABP to the users who should see that limited address book. Outlook 2019 may also need time to download a fresh offline address book, so changes will not always appear immediately.