Fire for cause means "you aren't doing your job properly."
Redundancy means "your job is no longer required."
If twitter restructures to save money, the jobs that no longer exist have been made redundant.
Maybe I misunderstood the point you were trying to make?
In US usage, the general term here is “layoff”. And severance is not a legal requirement, and practices vary considerably by industry and, within industries, by individual firm. Layoff is eligible for unemployment, while firing may or may not be. (The line is also looser because the US has at-will employment, so there are very little general rules–other than those in particular employment contracts of unionized or high-status employees–around termination other than for prohibited purposes, or–mostly around warning periods–mass terminations, but there are conditions for unemployment assistance.)