2004 Subaru Legacy died and lost steering in the left turn lane

So a bit of background, I’m not the original owner, it has about 190,000 miles on it, and is almost definitely due for an oil change. Anyways, I was driving home, the car was running fine, I got into the left turn lane and had to come to a complete stop in the intersection. As I was about to turn, I felt the steering lock up, and there was no throttle response, I was in a bit of a panic so I didn’t see if the engine shut off but several warning lights went on including the check engine light. I turned the key into the off position then restarted the car and it worked fine for the small remainder of the drive. Does anyone have any idea what caused this or how I could prevent it from happening again? It was very scary/dangerous.

Worn out ignition switch, and or more than 5 keys on your chain.

Barkydog is most likely correct, although at 190k, there may be other possible causes.
However, I would suggest that a worn-out ignition switch is the most likely cause.

The OP should be aware that when the engine stops running, the steering will lose its power-assist, and most–if not all–of the warning lights will light up.

The OP should also be aware that the car may be overdue for many types of maintenance, in addition to an oil change. For instance, unless it can be verified with hard copies of maintenance invoices that the timing belt was replaced w/in the last 100k miles, then it is due–or overdue–for that vital maintenance, which is supposed to be done every 100k miles. When that belt snaps, the cost of engine repairs may exceed the book value of the car.

In the absence of maintenance records, you have to assume that the car has had no maintenance, and you need to play catch-up right away.

Most likely your engine just died and you lost power steering. You still have the ability to steer but the effort will be quite high. Practice it some time in an empty parking lot.

As to why the engine died, it could be due to many things. What is the state of tune of your Subaru? Has it been idling rough or slow lately? Did you have the defrost ‘ON’ at the time?