Sadly some required packages don't seem to be up to date! The simplest way is to install the required packages from the repository manually until all requirements are there.
Go to https://slack.com and get their latest .deb install file for Linux
if you attempt to install it as, you should get some missing requirements warning and it bails.
sudo apt install <slack.deb>
failed: requirements missing
Head onto this debian repo, find and download the following file
http://http.us.debian.org/debian/pool/main/liba/libappindicator/
libappindicator3-1_0.4.92-7_amd64.deb (or newer!)
NOTE: if your system is not a 64bit intel, choose the approriate one (not the _amd64)
Unfortunately we need two more packages required by the previous one (down the rabbit hole we go)!
Thankfully it's only 2 more.
http://http.us.debian.org/debian/pool/main/libi/libindicator/
libindicator3-tools_0.5.0-4_amd64.deb (or newer!)
libindicator3-7_0.5.0-4_amd64.deb (or newer!)
Once downloaded, install them in the following order
sudo apt install <package.deb>
1- libindicator3-tools_0.5.0-4_amd64.deb 2- libindicator3-7_0.5.0-4_amd64.deb 3- libappindicator3-1_0.4.92-7_amd64.deb 4- slack(...).deb
And you should be good to go.