Intriguing question! I donned my deerstalker, stuffed my pipe and did some digging. The answer is:
apt does not archive packages.
The story goes back to Friday, 13 Sep 2002, when Daniel Quinlan filed bug report #160743, entitled "apt-get clean should run weekly". The rationale was that "The apt package cache grows without bound [...] runs the danger of filling up smaller disks, especially on systems run by less experienced Debian adminstrators."
The bug was closed 14 years later, on 2 Jan 2016, with little fanfare. The change isn't even mentioned in the changelog entry for apt 1.2. The commit message reads:
Author: Michael Vogt <email@example.com>
Date: Sat Jan 2 22:08:30 2016 +0100
Add new APT::Keep-Downloaded-Packages option
This option controls if downloaded packages should be kept after
a successful install or if they should be deleted. The default
for "apt-get" is that they are kept (just like before).
However the default for "apt" is that they get deleted.
So there you have it. I suppose that the introduction of the new end-user oriented
apt command (note that
apt-get is kept backward-compatible so as not to break scripts), was the perfect occasion for closing this historical issue.