Generally speaking, all the income should have been reported in 2020, although there might be a tax argument to delay reporting depending on what the escrow account was for and whether you had access to the money or not.
Also, the entire proceeds is not necessarily taxable, only your gain. Your gain is, roughly speaking, the difference between what you originally invested and what you received.
Was this a publicly traded company or a private company? Do you know what kind of company?
