I used weight painting tool to select specific area of an dental model object and I want to convert that selection to new separate mesh layer, is there any way to do so in blender 2.8?
I'm not exactly sure what you are after, but if you want to get a new, separate mesh of the teeth weight painted red, there's a simple way to do it.
In edit mode, you could Circle select (press C, mouse wheel changes brush size) and then "paint" the faces you want to select.
Then Shift-D to duplicate the selection. Then, P will Separate the selection into a new object.
Hope that helps.