DE-9 was used for MDA, CGA, EGA, and Hercules. Also some early VGA hardware used it, before DE-15 became sort of a de facto standard. (EGA had separate wires for red, green, and blue, they just carried digital signals. VGA used analog signals for red, green, and blue, so it was sensible to just map them to the same pins. Some monitors could accept either signal format and adapt to the timing of almost anything, the NEC MultiSync and Samsung Syncmaster for two.)
The Tektronics TDS-4xx oscilloscopes produced VGA-signals-on-EGA-pinout, for one example I've encountered in the wild.
DE9 connectors were also used for RS232C serial ports, which included mice and modems and all manner of other things speaking RS232. Wand-style barcode scanners were quite frequently on a DE9 connector, speaking some obscure protocol which may be at RS232C-compatible voltages but probably had positively Martian numbers of data/stop bits, etc. Or maybe just a raw photodiode signal so plugging into a serial port wouldn't do anything useful at all. Heaven help you.
Beyond that they were also used on Token Ring network cards (the hermaphroditic connector being to large to fit on a slot bracket), Fibre Channel in its copper variant (using the same pinout as Token ring, thank goodness they wouldn't blow each other up if mistaken), and pretty much anything else that only needed a few signals. It's a durable, ubiquitous, easy-to-work-with connector.
And of course in industry, the DE9 connector is still used for a zillion more things, like CAN. Pretty much every CAN test tool you'll find uses pins 2 and 7 for CANL and CANH respectively, with 3 and 5 as ground.
Pin 9 has quietly emerged as a sort of de facto phantom power supply, and if you stick 9-to-12-ish volts on it with a polyfuse, a lot of devices will run from that and not need batteries. Some RS232 GPS receivers would accept that, and the Toughbooks with serial ports still produce it.
Here's a funny: RS232 on DE9 is still produced today by the millions, on the backs of fancy TVs, because it's still the most sensible way to interface with high-end home automation systems. (USB never arrived at a good way to do peer-to-peer, and requires entirely too much software.) It's a 60-year-old standard and still has legs.