Disturbed did release their new album Evolution in 2018 though, and according to guitarist Dan Donegan in an interview with Guitar World, the EP's ideas were blended in with the album ideas and will not be released separately.
"We originally didn't know if we were going to release a separate EP of that or not. As we started to write for this album [last year's 'Evolution'], we started with the acoustic tracks. Once we had five or six really strong ideas, we went back to plugging in to get some signature Disturbed riffs going. As things started to develop, we decided to incorporate it all into one body of work instead of putting it out as a second EP. That's when we came up with the concept of 'Evolution' being the name of the album."
This makes sense given the amount of slower ballad-esque songs on Evolution. The album also debuted highly on the Billboard charts, so I guess people were still pretty into the record.