What happens in DESelect when a field in a source / target data extensions is deleted?

Field deleted from Selected Data Extension

The field will no longer be available under Available Fields within the Selection Criteria screen. 

In addition: If the field was used as part of a filter, DESelect will provide the user with a warning, and the filter line will simply be removed. However, the user can continue to view, edit, and run the selection (without this criteria). 

Field deleted from Target Data Extension

The field will no longer be available in the field mapping on the Target Definition screen. However, the user can continue to view, edit, and preview/run the selection.