At Amazon, when someone was laid off their Slack still worked for the 60 day WARN period. It was actually a problem because you would Slack them and get mad that they didn't reply. The only way to know for sure was to ask their manager, but you didn't want to do that because if they weren't laid off you didn't want to throw them under the bus!
That's effectively how it ends up, except with a slight advantage to the company. They cut you off and tell you that you don't have to work anymore, but in the off chance you get a job within 60 days, they don't have to keep paying you. They can also preserve their cashflow by not paying you up front.
But since you technically have to be "on the books", if something like Slack is tied to your status in the company directory, it's easier to just leave it.
Just an FYI - you can still get paid the WARN severance even if you take another job, just don't "quit" during warn. Your employment contract may or may not say you can't do side work, but (1) what, will they fire you? (2) it probably just says that you can't do work that interferes with your current employment, which is not a problem.
The WARN period exists to give you the money, but also keep you on for insurance and 401k vesting purposes (and similar). Getting cut off immediately, and suddenly losing insurance would be much much more disruptive, even with COBRA.
My org had a big cut last year but nobody would tell us how many people were laid off for some reason.
I happened to remember the total number of people who were in our org’s giant slack channel before the layoffs and thus was able to do some hardcore detective work subtracting the new number of people in the channel from the previous amount to get the answer…
Yeah, whenever I want to find out if someone still works at the company, I just search them on Slack. If it has "(deactivated)" after their name, they're no longer employed here.