Well, sometimes it was DBF files (of dBase / FoxPro / Clipper fame). The first app I ever wrote for money was a thing that prompted you to insert a floppy with a .dbf consisting of a list of recalled products and append it into a local database (another .dbf!) that would let the user query the whole thing. The floppies were obtained by means of a person walking to the local office of the corresponding government entity where they would hand a blank floppy and receive it back with the copy of the most recent updates. That was late 90s.
This evolved through multiple iterations as the government end of it did. At one point you no longer had to walk there physically; instead, there was a dial-up endpoint running UUCP that could be used to fetch the file. Then they found out about Internet, and it became a website with the file, now .mdb (MS Access) rather than .dbf. I rewrote the app in .NET/WinForms, since it was much easier to do all those things from it than to try bolting it onto an old TUI DOS app.
But it was still basically the same table with the same schema by mid-10s, and my app was still chugging along. I wonder sometimes if some iteration of it is still in use today; wouldn't be surprised if it were.