It looks to me like this is a bug - maybe in the Customization Wizard, or the AcroRead.msi.
I noticed on my install that bEnableADCFileStore was set to 1. Deleting the value entirely (or setting it to 0) removed it from the Welcome Screen and the File Menu.
I am experiencing the same issue.
Any more ideas how this could be managed, this occurs on fresh installs as well as upgrading previous products.
I have found that setting the following registry key removes the Sign panel and the link from the Online Features menu on application launch. Just need to locate the setting to remove the Adobe Online Services menu from startup too now.
HKLM\SOFTWARE\Policies\Adobe\(product name)\(version)\FeatureLockdown\cServices\ DWORD: bEnableSignPane = 0