all shares lost on a reverse split

Yes, you can declare a loss. Here is how to report.

1. Open (continue) your return in TurboTax.
2. In the search box, search for investment sales then click the "Jump to" link in the search results.
3. Answer Yes to the question Did you sell any investments?
4. If you land on the Here's the investment sales we have so far screen, click Add More Sales.
5. On the next screen, select the type of sale you had (second home, collectible, land etc.) and click Continue. here you would say stocks and bonds.
6. Continue following the onscreen instructions to enter the sale.
7. When it asks you for the sales information, you will enter what you paid for the stock and then enter zero for sales proceeds and then all other pertinent information that it asks.
8. The next screen should ask Let us know if any of these situations apply to this sale
9. Here you would check This was a worthless security, which is what it is since you lost your shares.

Thank you. Do I need to show proof? I didn’t receive anything from my broker but I do have the legal documentation showing the reverse split

Not when you file.  But make certain that you save everything in order to show the IRS if they have questions later.

@dfinn58