If the departures are such that for each team, 1 competent senior engineer remains who can keep that domain running, then Twitter could be unaffected with 1/5 of its workforce. They might not be building new features, but the current site could function, which is reasonable since the current site is arguably feature complete.
But in reality it won't be as clean as that. If the whole localization team quits, suddenly the site won't be translated to other languages. If the whole mobile release team quits, suddenly the Twitter app might disappear from the app store. Or the security team is gutted and suddenly everyone's passwords are leaked.
By all accounts after the layoffs, voluntary resignations in response to layoffs, and people who refused the ultimatum and so probably consider themselves laid off, it's not clear Twitter even has a skeleton crew
But the current reports are in the vicinity of 60% to 90% of the companies employees.
Obviously it’s extremely hard to get accurate numbers because one minute it’s reply yes to this or your gone, the next it’s report to this room, then batches are getting rehired because they are essential to operations and it’s jumping around like the staff head count graph has arrhythmia.
Someone at Twitter knew what they were doing. Let's hope they didn't get swept up.
Side note: do we know how much Twitter Sr. Devs make for sure? I know the term "400k for 5hrs a week" keeps getting thrown around. However, that doesn't seem incredibly well researched and has some triggers baked in