Using BetterSnapTool, the drag-to-snap no longer works in Outlook since upgrading to macOS Catalina. It seems to be drag-to-snap alone that's not working. I can re-size Outlook using the window position/size menus, and by using keyboard shortcuts. Other apps work for all three methods.
This is with BetterSnapTool 1.9 and Outlook 16.29 (19090802), macOS 10.15 (19A583) on a 15"MBP, A1990.