Where are TWRP backups stored on SD card? Had to un-root I had to un-root my LG G2 Verizon because recovery got corrupted. I created a TWRP backup some time ago from within its recovery and it was stored on the internal drive. I did a system reset, and then un-rooted by going back to stock image. Did the system reset delete that backup file from my SD card? If not, what location is it under?
Flashing the stock image may have erased your backup. However, since the G2 supports an External SD card, I don't think it would have.

From the SD Card's root, it should be at ./TWRP/BACKUPS/. Restoring a backup from here using TWRP will re-root your device (If your device was rooted when you made the backup). You can also use apps like Titanium Backup to selectively restore parts of it... but Titanium Backup requires root.

You may, however, be able to selectively pull files from the backups - a full backup of your phone's filesystems.