When updating using apt-update, you get the following warnings:
$ sudo apt update
[sudo] password for matthew:
...
Hit:10 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
Fetched 9,870 B in 1s (7,957 B/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see them.
W: https://packagecloud.io/slacktechnologies/slack/debian/dists/jessie/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.One way to resolve this is to export the GPG key from the deprecated keyring and store it in /usr/share/keyrings. Fortunately, it's not too difficult:
$ sudo apt-key list
[sudo] password for matthew:
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 <support@sublimetext.com>
sub rsa4096 2017-05-08 [S]
pub rsa4096 2014-01-13 [SCEA] [expired: 2019-01-12]
418A 7F2F B0E1 E6E7 EABF 6FE8 C2E7 3424 D590 97AB
uid [ expired] packagecloud ops (production key) <ops@packagecloud.io>
pub rsa4096 2016-02-18 [SCEA]
DB08 5A08 CA13 B8AC B917 E0F6 D938 EC0D 0386 51BD
uid [ unknown] https://packagecloud.io/slacktechnologies/slack (https://packagecloud.io/docs#gpg_signing) <support@packagecloud.io>
sub rsa4096 2016-02-18 [SEA]
...
# `038651BD` is the last eight digits of `pub` key signature for slack
$ sudo apt-key export 038651BD | sudo gpg --dearmour -o /usr/share/keyrings/slack.gpg
# Add the "signed by" tag to the existing .list apt source file,
# corresponding to the output path from the previous command
$ sudo cat /etc/apt/sources.list.d/slack.list
deb [arch=amd64] https://packagecloud.io/slacktechnologies/slack/debian/ jessie main
$ sudo echo "deb [arch=amd64 signed-by=/usr/share/keyrings/slack.gpg] https://packagecloud.io/slacktechnologies/slack/debian/ jessie main" > /etc/apt/sources.list.d/slack.list
# Confirm the warnings are gone
$ sudo apt update
...
Hit:10 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
Fetched 9,870 B in 1s (7,957 B/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see them.
# Delete the original key
$ sudo apt-key del 038651BD
[sudo] password for matthew:
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
OK