Not having "ListenStream=631" in the systemd cups.socket file is
intended. See the answer from Mike Sweet, author of CUPS:

First, the reason why we don't include "ListenStream=631" is because that 
exposes cupsd to remote attack. And we don't listen on localhost (which has a 
similar security issue) because it causes problems when binding to the "any" 
address on Linux.

Second, once cupsd starts it will create the localhost (or "any" address) 
listener for you, and stay running indefinitely. And cupsd uses a KeepAlive 
file to tell systemd to start it up on bootup once you enable the web 
interface, so it should be running right away unless you've added something to 
manually start/stop cupsd.

In Yakkety (16.10), CUPS behaves as intended upstream, not automatically
shutting down when it shares at least one printer or the web interface
is active (turned on via "WebInterface Yes" in cupsd.conf). So there
this bug should be fixed.

In earlier Ubuntu versions, like Xenial (16.04 LTS), there is a patch
which allows for CUPS auto shutdown due to another problem, whgich is
fixed in later CUPS versions. This fix needs to get backported so that
auto shutdown can be suppressed with active web interface, to match the
upstream standard.

** Changed in: cups (Ubuntu)
       Status: Confirmed => Fix Released

** Also affects: cups (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Changed in: cups (Ubuntu Xenial)
       Status: New => Confirmed

** Changed in: cups (Ubuntu Xenial)
    Milestone: None => xenial-updates

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  cups hang after a while

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