Disclaimer: Not a CPA or Attorney
One thing to note is that if you have maxed out on social security in 2021 then your severance won't be taxed the 6.2% in 2021 on any of the payment. You will paid the Medicare 1.4% regardless of year and income (no phase out)
In Florida, with no state income tax, it could be beneficial whereas California's graduated income tax would probably be a ki---r if the severance is substantial.
Project out possible income in 2022 (planning time off or getting back to work at a better employer as soon as possible?) and look at your income projections for 2021 which should be pretty baked at this point. If you have a lot of room in the 24% or below brackets then 2021 might make sense.
Note that Capital Gains/Investment income if applicable and substantial should be factored. There is a 3.8% Investment tax (part of the Obamacare funding tax) above certain income levels depending on single or married cap levels.