Old employer sent me a 2019 1099-misc, claiming I got income in 2019. I did not. Asked they fix this

Did you not bring this to their attention earlier this year when you got the form so it can be corrected ?  What have you don't do date ? 

Sounds more like a legal matter to me. Might want to get a lawyer or you yourself request they provide proof that you received payment for anything from them in 2019. I would think they can not prove a transaction that never occurred.  It won't be on their own bank racords anywhere - at least not on the banks records which only the bank has access to.

If you receive an incorrect 1099, first reach out to your client and try to get it corrected. If that doesn’t work contact the IRS at (800) 829-1040 for instructions. You may just have to include a letter with your tax return stating why there is a discrepancy.