Hi!
I am glad you asked about the e-file. I was thinking about my answer and I should have added that you will probably have to mail the 2019 return if you add the statement. I also noted the person who said to put zeros in as a way of acknowledging the 1099-B that you received. I would like to build on that thought. Using zeros on Schedule D does work, and if the 1099B has an amount on it, you could simply use the amount reported as both the cost and sales price, resulting in a zero gain or loss for this year. That would satisfy the IRS for this year without an additional statement, and you would have received your loss last year, so everyone is happy.
As far as who gets to decide, an event needs to happen that shows it is worthless. So any news that the company is in dissolution or filing for bankruptcy would be grounds for you claiming the loss. It is good that you didn't do it in 2015 when you just felt it was worthless. I'm presuming that in 2018 you had heard of the bankruptcy. Even if you didn't, I still do not believe that you should amend 2018. If it is ever questioned by the IRS and the loss is disallowed, you will have opportunity to then amend the 2019 to re-claim it.
