@thomzane I inspected an older version and soon found proprietary software, but I now inspected hplip-3.23.5 and found proprietary software as well.

Sure COPYING says:
HPLIP is free, open source software, distributed under the following open source licenses:

GNU General Public License (GPL) v2
MIT license
BSD license
GNU General Public License (GPL) v3

Then I read down and well: "A small subset of HP devices require proprietary software technologies to allow full access to printer features and performance. These technologies cannot be open sourced. Because of this, HP is releasing binary plug-ins for each of these printers that work in conjunction with our Linux Open Source Printing Software to improve the printing experience for HP�s Linux Printing Customers. These binary plug-ins require the user to read and agree to a license agreement at the time of driver installation.", which makes it pretty clear that there's proprietary trickery at play.

Under ppd/hpcups there are lots of proprietary ppd files, but I guess that's not software.

hplip contains at least 2 proprietary binaries under;
hplip-3.23.5/prnt/hpcups/libImageProcessor-x86_32.so
hplip-3.23.5/prnt/hpcups/libImageProcessor-x86_64.so

https://bugs.launchpad.net/hplip/+bug/1785230

I'm sure there's many more issues but I need to check my older analysis first.


There's also a bunch of proprietary plugins that they claim to be separate, but are clearly from a derivative work: https://developers.hp.com/hp-linux-imaging-and-printing/plugins

Pretty much either;
a. HP holds all the copyright and says the it's under the GPLv2-or-later, but is lying and using the GPLv2 and v3's good name so people don't realise it's proprietary.
b. HP has made derivative works of existing GPLv2 and GPLv3 works and has committed copyright infringement by infringing the license on such works by making proprietary derivative works, but has avoided committing clear cut infringement by ensuring to release the existing source files under the same license.


Any recent printer supports IPP{S}, so I recommend using that instead.