From the article:

a lawsuit was filed in the US federal court in San Francisco seeking orders for Twitter to comply with the federal Worker Adjustment and Retraining Notification Act, which requires 60 days’ notice for mass sackings at large employers.

https://en.wikipedia.org/wiki/Worker_Adjustment_and_Retraini...

Edit: Here's the complaint: https://storage.courtlistener.com/recap/gov.uscourts.cand.40...