SharePoint Server Subscription Edition AMA

Hi Henrik, yes, SharePoint Server Subscription Edition can be installed, run, and updated in an environment with no Internet connection.  You'd be responsible for ensuring the appropriate software prerequisites are installed since the SharePoint Prerequisite Installer wouldn't be able to download them for you.  You'll also be responsible for downloading installing the monthly updates for for SPSE from https://aka.ms/sharepointupdates.

That said, functionality that depends on an Internet connection wouldn't be available.  For example, no hybrid functionality would work since that requires an active Internet connection to your M365 tenant.  You also won't have access to most of the online help articles in the built-in SharePoint Help feature since those are hosted on the Internet.  And if you're using OneDrive Sync client to sync your documents, it would fall back to polling the farm for updates instead of receiving push notifications whenever an update occurs.