Why the Slack app is not in the "Recent Apps" list? My phone: Samsung S7 Edge
OS: Android 7.0 Nougat

Almost all the apps can be shown in the recent apps list, which can be seen by pressing the left soft touch button, except the Slack app.
Some apps can hide themselves from Recent list.

There's nothing wrong. Slack is hiding itself. There's no solution except contacting its dev team.