The W-9 request is legitimate. The partnership would need it to properly report your payout either on a Form 1099-NEC or final Schedule K-1.
The buy-out itself isn't generally income. Instead, you'll need to determine if you have a gain or loss on the liquidation. To calculate that, you'll need to know your "basis" in the business (your after-tax investment + your share of the profits less any distributions you received over the years - assuming this was a partnership LLC).
If the payment was less than your basis, you'd report a loss.
If the payment was more than your basis, you'd report a gain.
This would typically be reported on Schedule D and Form 8949 as a capital gain or loss, much like selling any other investment.
Your 2023 Form K-1 might affect your final basis calculation, so you'll want to have that available.
