I apologize if this question was answered before (I searched Slackware forums to no avail).
In Slackware-current 64-bit updated on 03-01-2022 with custom kernel 5-16-12 and cgroups enabled I have Applications Panel in System Monitor empty with the info: "Applications view is unsupported on your system"
I have found few tips on the web:
1) custom kernel at fault (CGROUPS not enabled, but I have CGROUPS enabled).
2) available easily in SystemD enabled distros or need to do some work.

Is there a way to get System Monitor Applications working in
Slackware?

Thank you

Apparently, those "applications" are a feature of systemd based distributions where the programs and their children programs are all executed in particular cgroups tracked by systemd.

A classical example I seen is a Firefox instance which spreads on dozens of children instances, which represents a single "application" for systemd.

Obviously, this feature is not available on a non systemd based distribution like is Slackware, so probably is better to ignore it and those pages from Plasma5's System Monitor.

Concur that Plasma System Monitor, Application Page needs systemd to work.

You can hide the Applications page. Click on hamburger menu, Edit or Remove pages..., uncheck Applications.

Thank you both. This is what I gathered from the web.

Thank you @chrisretusn for the tip.