Companies pay Slack based on the number of employees they have. Because many companies have performed layoffs, they're paying less for Slack. So, Slack's revenue has gone down, so they have to perform their own layoffs.

This is in line with the reported 10% slashing of Salesforce in general.

Given software salaries are pretty high why not cut 10% of comp?

Unless they just wanted to fire them and just using this time as an excuse.

Can they cut 10% of their upper management team too?

I've always wanted to ask that since a company I worked for did a widespread 10% headcount cut. I survived it, but left soon after, surmising that the company wasn't going to be around for much longer.