You can still impose disadvantage against saving throws, within the mechanics as is, most notably would be Hex.
Hex doesn't (or at least shouldn't) impose disadvantage on saving throws. It imposes disadvantage on ability checks (i.e. skill checks and straight ability checks but not saving throws which are separate).
The disadvantage factor is intended to give some out of combat utility and it's in combat use is fairly limited (grapple checks, checks imposed by some types of terrain, some small number of spells include an ability check rather than save (Control Water, Detect Thoughts, Phantasmal Force and Telekinesis for example)).
