If a proposal is not taken over in the leading terminology system outside of translate5, it can not be detected, why it is not present in the current import:

• because the proposal is not decided yet
• or because it is rejected.

The easiest and safest way to solve this is to introduce for PMs and admins and API an option to delete a term or an attribute.

Therefore for these roles a red cross icon is introduced next to the pencil icon, that lets you propose changes. A click on the red cross triggers a pop-up asking "Do you really want to delete the attribute XY / the term XY"? And on yes does the deletion.

As an alternative the import flags 

• deleteEntriesModifiedOlderThan=2018-05-29
• deleteEntriesOlderThanCurrentImport=true

can be used on import time with the termImport plugin. They are also added as import flags in the import window of the GUI.