A Label node has shadow_offset_x, shadow_offset_y, and shadow_as_outline attributes that can be set from the inspector or in a custom theme.
RichTextLabel has no such settings in the inspector or the theme editor, which suggests to me that these attributes do not exist for RichTextLabel, or, at the very least, they can’t be changed. But I’m not certain.
Thanks for your answer even though little disappointed because it ads nice effect to the font. I might end up with label if that is going to be enough.