I'm not sure why this is such a common take. Many functions in a typical business, overstaffed or lean, could be greatly pared back without an immediate impact. UX, UR, product, legal, HR, design, recruiting, infosec, analytics, marketing, sales, facilities, etc. The question is will it have a negative impact 6 or 12 or 24 months down the road.
I have no personal knowledge on whether Twitter was overstaffed but the "we got rid of X, Y, Z and the business hasn't collapsed yet so must not be needed!" is a bad take.
People quit all the time and they get replaced. It's the same thing with layoffs. Another thing is that a lot of positions are filled by people that are over educated or experience for the job they hold. They do it simply because that person wants to work for a FAANG and are open to getting any position they can. My suspicion is that a lot of people are going to move jobs as jobs become available and some positions will be merged from say 2 to 1. I'm sure it's crazy now but they will be fine.