Need to be able to disable taxonomy term, but maintain legacy
Depends where you wish to add the term. If you need to add it to a Node creation page, you could always, instead of adding the field as a term reference, add it as an entity reference, select Taxonomy Term as target, select the correct bundle, and in ENTITY SELECTION, select Views: Filter by an entity reference view. 

In order to do that you will beforehand need to create a view on the taxonomy terms that you wish to display and hide (ie filter out those terms that should not be displayed), then add this view to the dialog that will appear when you select Views: Filter by an entity reference view