Thanks for the answer!
To clarify your explanation with an example: in 2019 I had 100 units vested at $10. The STC was 30 units. So my 2019 W-2 reflects the $1000 income, and $300 paid income tax. I have 70 units remaining. So far so good.
Then in 2021 I had 200 units vested at $10. The STC was 60 units. So my 2021 W-2 reflects the $2000 income and $600 paid income tax.
In addition in 2021 I sold the 70 units left from my 2019 RSU, at $10. My 1099-B has two transactions:
60 units at $10, from the 2021 STC
70 units at $10, from the 2019 left over
The 60 STC has a $10 cost basis, since it was sold immediately at vest. Let's assume no transaction fee, so net $0 gain.
The 70 units I sold at $10. Since the vest price was also $10 then no net gain, but then is the suggestion to include the 30 * $10 from the STC in 2019 in addition, as cost basis?
Overall I can understand the need to keep all documents. And if it's a matter of "can I track the RSU income and taxes paid" in previous years, and so that if I present all the W-2s from all relevant years to back up the income and tax paid as part of RSUs, then yes I can track them. However Turbotax seems to indicate it need my current year (2021) W-2 to be able to track against sales of previous year RSUs, which at least for my employer they don't do.
