My 94 year old mother died 2-11-19. Her 2018 Adjusted Taxable Income was less than $3,000 but I didn't know if she was required to file. On filing day I lacked one 1099 so I mailed a Turbotax extension form to the IRS. I recently read that I should have received an IRS acknowledgement of the extension but I never received one. Today I sent the completed the 2018 taxes to the IRS. If the IRS somehow never received the extension that I mailed will I be penalized>
The IRS does not acknowledge mailed extension forms. They only contact you if they do not grant the extension.
It's possible that there would be a penalty for late filing if the IRS did not receive the extension, but if there was no tax due on your mother's tax return, there is no penalty for filing late.
Sorry for your loss. When you mail in an extension you do not receive a notice that the IRS received it. You receive an accepted notice when you e-file an extension. When you are due a refund or don't owe taxes there is no penalty for filing a late return. If taxes are owed, then they needed to be paid by April 15, 2019 to avoid any penalty and interest. If taxes are owed the penalty & interest accumulate until the taxes are paid in full.