Debian wasn’t the first distribution to separate the changelogs, but it might have been the first to maintain per-package changelogs.
Changelogs for software have existed for a long time (they’re a fairly obvious variant of lab notebooks or engineering logs). Early Linux distributions kept their own changelogs in a similar fashion; since they logged changes made to the distribution, they were naturally kept separate from the changelogs of the software being distributed. For example, SLS had a changelog with entries like
920901: Initial release (.96p4). Didn't use jump tables though.
921007: Release .98p0. Jump tables used.
921011: menus.taz: new sysinstall and sysbuild.
a1: New .98p1 image with new sysinstall
a2: New rootdisk
image.taz: 0.98p1 kernel, Fixed top.
devs.taz: modified /dev directory
image.taz: Linux .98p1 image, ps, etc.
lx96p1.taz: Linux source for .98p1 (replaces lx98.taz).
tcpip.taz: Telnet and fpt 2.2.2 binaries.
c4/: Added new disk, clisp to compiler series.
x*: Reorganized and fixed double compressed fonts.
The earlier MCC also had changelogs, for example this one for 0.99p8.
Users familiar with Slackware will find the format familiar; Slackware still maintains distribution-wide changelogs.
For both SLS and Slackware, a single distribution changelog made sense because they were both initially the work of one person (Peter MacDonald for SLS, Patrick Volkerding for Slackware) and changes were thought of in terms of the whole distribution, not individual packages (at least, not to the same extent as nowadays).
As you found out in the mailing list discussion on
debian-devel, the current changelog practices in Debian weren’t present right from the start. In early 1994, Debian 0.91 had a distribution-wide changelog and its packages don’t contain a changelog (at least, not its binary packages). By late 1994 and Debian 0.93, source packages had individual changelogs, kept in
debian.README (which also contains licensing information); see LILO’s packaging patch for an example:
9-July-1995 Bruce Perens <Bruce@Pixar.com>
Upgraded to version 16.
23-December-1994 Bruce Perens <Bruce@Pixar.com>
Added Debian GNU/Linux package maintenance system files, and configured
(Note that looking at packages such as
dpkg or the Debian documentation, as suggested in the mailing list, isn’t all that useful since they are native packages, with a single changelog covering software and packaging changes.)
So it seems Debian was the first Linux distribution to store per-package, distribution-specific changelogs, separate from the upstream changelogs. Other distributions followed suit, at least partially, with e.g. RPMs storing changelogs in their spec file (albeit typically with less detail than Debian changelogs, and only in the source package). There might be similar practices in the BSD world, I haven’t checked; and I can’t remember what packaging initiatives, if any, were around for other Unix systems in the early nineties.