Can I bury a soursop plant in the ground in a 20 gallon (91 liter) pot? I don't want it to grow to a 15 to 20 feet height tree because it would be too tall in the back yard.
Well you could, but the only way it would grow healthily after a year or so is if it put roots out through the bottom of the pot into the ground, which it is very likely to do. The roots will come out through the holes in the pot and eventually break the pot as they increase in size. You can't stop the holes up because then there'd be no drainage for the roots. So on balance, my answer would be no, its not a great idea. If you are you growing it for its fruit, which it usually produces 3-5 years after its been planted in the ground, this might indicate it needs to be a certain size before it fruits, and it may not attain that size in a pot, even a 20 gallon one.
My soursop has been in a pot for 3.5 years since seed, 40L in size, will be repotting it shortly as its now 7foot tall, it's fruited once and has been pruned once decently. These trees have a shallow root base so if you can provide for that and have a stable base, you just need to make a healthy growing environment and it will do well for a while, long enough for you to explore options at your leisure for the future.