Apt-key deprecation warning when updating system

Error

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
8 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: https://packages.microsoft.com/repos/edge/dists/stable/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
W: https://download.sublimetext.com/apt/stable/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

Solution

1. List existing keys:

   $ sudo apt-key list
   
   Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
   /etc/apt/trusted.gpg
   --------------------
   pub   rsa4096 2017-05-08 [SCEA]
         1EDD E2CD FC02 5D17 F6DA  9EC0 ADAE 6AD2 8A8F 901A
   uid           [ unknown] Sublime HQ Pty Ltd <[e-mail]>
   sub   rsa4096 2017-05-08 [S]
   
   pub   rsa2048 2015-10-28 [SC]
         BC52 8686 B50D 79E3 39D3  721C EB3E 94AD BE12 29CF
   uid           [ unknown] Microsoft (Release signing) <[e-mail]>

2. From here, export a key:

   sudo apt-key export BE1229CF | sudo gpg --dearmor -o /usr/share/keyrings/microsoft.gpg

   Note: The BE1229CF value comes from the last 8 characters of the pub code.

   The following message will likely appear:

   Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).

3. Now we can update our apt source file for the repository (e.g., /etc/apt/sources.list.d/microsoft.list), adding a signed-by tag:

   deb [arch=amd64 signed-by=/usr/share/keyrings/microsoft.gpg] https://packages.microsoft.com/repos/edge/ stable main

4. Update apt to confirm the message is gone:

   $ sudo apt update
   
   Reading package lists... Done
   Building dependency tree... Done
   Reading state information... Done
   All packages are up-to-date.
   W: https://download.sublimetext.com/apt/stable/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

5. Remove the original signature:

   sudo apt-key del BE1229CF

This can be done with each of the warning messages. Once done, apt will no longer complain.