Currently, if you use the “Improved Enum Drop Down” you can only get an “All” option in Odin Inspector by adding an actual “All” enum entry with a value (-1 or 255). This is not ideal, as this isn’t by itself a valid enum entry.
It would be great if you got the same features as with the “standard” Enum Flags Inspector.
With the “standard” (non-improved) Odin Inspector:
With Improved Odin Inspector:
With the basic Unity Enum Flags Inspector: