*) As for priority:  It's not as low as you might imagine.
The problem affects not just ifconfig but also ifup, which is
indispensable.  It also affects the "raise network interface"
target at boot time, as triggered by "auto dummy0" directives
in /etc/network/interfaces.

Symptom:
:; ifup dummy0
Cannot find device "dummy0"
Failed to bring up dummy0.

This was hinted at in the original report but not emphasized.
I emphasize it now.  I mentioned ifconfig because it is the
/simplest/ way to exhibit the bug.  Feel free to change the
headline of this bug to "ifup" (rather than "ifconfig") and
transfer it to the ifupdown package (rather than net-tools)
if you wish.

This bug means that when I upgraded my machine remotely, it
became unreachable, because the dummy0 interface carried the
stable IP address that I was using.  It took a great deal
of work on my part to devise a workaround.

*) In answer to your question:  "until recently" means until
I upgraded from xenial to bionic.  I have another system still
running xenial where ifconfig, ifup, and "raise network interface"
all continue to work fine.  The kernel version is exactly the
same, 4.19.42;  this may help you determine whether the
regression is in user space and/or kernel space.

On the other side of the same coin, I have yet another system
with a squeaky clean bionic (installed ab_initio, not upgraded)
that exhibits all the same problems with ifconfig, ifup, and
"raise network interface".  So it appears to be a bionic issue.

I have no easy way to test the releases that came between
xenial and bionic.  All I know is that bionic is the latest
LTS.  It's supposed to be supported, and it's broken in a
way that caused me a lot of grief.

*) FURTHERMORE, there is nothing in the ifconfig man page to
suggest that it is in any way deprecated.  What's the deal?
Choose your farce:
-- Is it on double-secret probation? (Animal House)
-- Doesn't it defeat the purpose of a deterrent if
  you keep it a secret? (Dr. Strangelove)
-- Is perhaps the notice on display in the bottom of a
  locked filing cabinet stuck in a disused lavatory with
  a sign on the door saying "Beware of the Leopard"?
  (Hitchhiker's Guide)

If it really is deprecated and unsupported, somebody
should file a separate bug against the documentation.

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https://bugs.launchpad.net/bugs/1828749

Title:
  ifconfig dummy0 : Device not found

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