`apt` doesn't remove dependencies that were automatically installed

Question

System : Ubuntu 18.04

apt doesn't remove jackd's dependency jackd2 (and its own dependencies) after uninstalling jackd :

$ sudo apt install jackd
The following additional packages will be installed:
  jackd2 jackd2-firewire libconfig++9v5 libffado2 libglibmm-2.4-1v5 libsigc++-2.0-0v5 libxml++2.6-2v5 python-dbus python-gi qjackctl

$ sudo apt remove jackd
The following packages will be REMOVED:
  jackd qjackctl

$ sudo apt autoremove 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 15 not upgraded.

$ sudo apt remove jackd2
The following packages were automatically installed and are no longer required:
  libconfig++9v5 libffado2 libglibmm-2.4-1v5 libsigc++-2.0-0v5 libxml++2.6-2v5 python-dbus python-gi
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  jackd2 jackd2-firewire

$ sudo apt autoremove 
The following packages will be REMOVED:
  libconfig++9v5 libffado2 libglibmm-2.4-1v5 libsigc++-2.0-0v5 libxml++2.6-2v5 python-dbus python-gi
0 upgraded, 0 newly installed, 7 to remove and 15 not upgraded.

Answer 1

There may be another package installed on your system that recommends or suggests the jackd2. You can use aptitude why jackd2 to find that out.

In my case it's the blender package through another dependency:

$ aptitude why jackd2                                                                                      
i   blender          Depends  libjack-jackd2-0 (>= 1.9.10+20150825) | libjack-0.125
i A libjack-jackd2-0 Suggests jackd2 (= 1.9.12~dfsg-2+b1)                          

And your apt is probably configured to treat recommendations and suggestions as dependencies, the settings for this are called APT::Autoremove::RecommendsImportant and APT::Autoremove::SuggestsImportant.

I'd recommend to leave the first setting alone, but you can safely disable the suggests setting (but it may still uninstall some packages that you want to keep, unmark them as automatically installed in that case).

To disable the settings that treats suggests and depends with autoremove:

sudo apt-get autoremove -o APT::Autoremove::SuggestsImportant=0