As the question in subject suggests, the membership page is missing. 
 
There used to be a link to a membership page which stated that my Product is Sonar Platinum and it lasts till infinity.
 
Where's this page gone ? 
 
 

Hi. I just noticed the same. Might be some reconstruction/maintenance going on.
All the best.

Conspiracy theory - SONAR is being rebranded as RANOS, and no one has membership (yet).

mettelus
Conspiracy theory - SONAR is being rebranded as RANOS, and no one has membership (yet).

Yeah....RANOS. 
I want the MUNITALP version...

If you go to your account for "My Store Activity":
 
 
https://www.cakewalk.com/My-Account/Store-Activity
 
and look at the latest "Sonar" purchase (i.e. Platinum) - it will show you the purchase of "Sonar Platinum with Lifetime updates" . Clicking on link takes you to the product page:
 
It shows membership ends on "infinity" (if you have lifetime updates)
 
 
Hope that helps - I know it does not explain about the "membership page", but it does get you the info you are asking about . . .
 
Syphus