Maybe I'm wrong in the aggregate, but what I have been told by a large number of friends and acquaintances is that housing costs near central Austin are very high. This was confirmed by me just trying to look for a place near campus on apartments.com. Other people who live in the city are able to dodge these costs by avoiding central Austin, working remote, or in Georgetown/Round Rock, but if you try to dodge these costs by living far out as a grad student then you are eating other costs in transportation, parking, etc. One thing that is important to keep in mind about Austin as a city is that there is no effective public transit infrastructure. The buses are unreliable, there is no subway or similar system, and the highway system in Austin is a complete mess. I say all of this only to communicate how important it is to have money in the city.
Maybe it is possible to make it work in Austin on 20k, but it is certainly not comfortable. My sense from the initial impression was that Austin was being lumped together with other small towns in Texas or rural areas as if the costs of living are similar. I know of programs which are much lower ranked than UT Austin and in much, much cheaper cities that have a higher stipend. Graduate students at UT have been trying to organize for some time because their situation is not livable.