regarding NAS certification E20-538

Yes, You will get certified as EMCSA Specialist for NAS (as you are already associate certified). Regarding exam retirement, I am not seeing any notification for this certification for now. EMC Proven Professional team will be able to explain you more on this.

EMC Employees: For information on the EMC Proven Professional Program : for practice test, exam descriptions and recommended study materials:

1.Log into  Education Portal  ( http://education.emc.com/ ); Select  Certification  
2.Scroll down (middle center of screen) to the  Appropriate Track  you are pursuing (i.e. Tech Architect); Click on the  Specialty  you wish to pursue within the track (i.e. CLARiiON Solutions, Symmetrix BC)

Non-EMC Employees:  For information on the EMC Proven Professional Program : for practice test, exam descriptions and recommended study materials:

1.Log into the  Education Portal  ( http://education.emc.com/)
2.Select  Certification;  Select  Certification Framework  (found on the left hand side or middle of the page) 
3.Click on the Track you are pursuing (i.e. Tech Architect); Click on the Specialty you wish to pursue within the track (i.e. Solutions, Symmetrix BC)

The document of Top 10 FAQs from the EMC Proven Advisor Team is for your reference.

To receive a Specialist certification, you must first complete an Associate level certification. Since you have completed ISMv2 (E10-001) already, if you pass the E20-538 exam, you will achieve the Specialist level certification.