I've been facing this myself, it's indeed annoying. I didn't find a perfect solution, but I did find a workaround: make the slack.list repo file (after adding signed-by entry to it) immutable:
sudo chattr +i /etc/apt/sources.list.d/slack.list
The biggest inconvenience here is that add-apt-repository fails with an error due to this immutability:
$ sudo add-apt-repository ppa:ubuntu-toolchain-r/test PPA publishes dbgsym, you may need to include 'main/debug' component Repository: 'deb https://ppa.launchpadcontent.net/ubuntu-toolchain-r/test/ubuntu/ lunar main' Description: Toolchain test builds; see https://wiki.ubuntu.com/ToolChain More info: https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test Adding repository. Press [ENTER] to continue or Ctrl-c to cancel. Adding deb entry to /etc/apt/sources.list.d/ubuntu-toolchain-r-ubuntu-test-lunar.list Adding disabled deb-src entry to /etc/apt/sources.list.d/ubuntu-toolchain-r-ubuntu-test-lunar.list Traceback (most recent call last): File "/usr/bin/add-apt-repository", line 362, in <module> sys.exit(0 if addaptrepo.main() else 1) ^^^^^^^^^^^^^^^^^ File "/usr/bin/add-apt-repository", line 355, in main shortcut.add() File "/usr/lib/python3/dist-packages/softwareproperties/shortcuthandler.py", line 221, in add self.add_source() File "/usr/lib/python3/dist-packages/softwareproperties/shortcuthandler.py", line 307, in add_source sourceslist.save() File "/usr/lib/python3/dist-packages/softwareproperties/extendedsourceslist.py", line 391, in save with open(filename, "w") as f: ^^^^^^^^^^^^^^^^^^^ PermissionError: [Errno 1] Operation not permitted: '/etc/apt/sources.list.d/slack.list'
So if you have such an error, just make slack.list mutable again, run add-apt-repository, and back to immutable:
sudo chattr -i /etc/apt/sources.list.d/slack.list sudo add-apt-repositor <smth> sudo chattr +i /etc/apt/sources.list.d/slack.list
