They didn't phase it out like they do these days. I had surgery 15 years ago. I was counseled weekly by a nutritionist. And I was enrolled in a 7-year university study that contributed to the bariatric journals, which improved the entire process. I think having a nutritionist is essential. They took so much blood I thought they were secretly vampires. They hospitalized me a few times for multiple-day studies. So I was part of the development of the procedures today, but they did major surgery on me and I still have the 7" scar to prove it.
Bariatric surgery is so much easier today than it was then, and since before I had surgery, when thousands of people went through much more exhaustive procedures than I was. I was in hospital for three days, and bed at home for two weeks.
Someone mentioned constipation in an earlier thread. I cannot stress the importance of taking in fiber laxatives before and after the surgery. The story is too gross to relate here but my first bowel movement took almost two days, with a giant stool developing for days giving me extreme pain. No enema could defeat it.
Prepare yourself physically by simply drinking fiber laxative three or four times a day. This isn't voodoo. We do not, DO NOT, create enough fiber after surgery because that's most accessible by eating vegetables, which is difficult to do when on a liquid diet. I did find salvation in V8 juice, but it was a hard process.
But let me stress again, it's NOTHING like that these days. It's usually day surgery and there's very little discomfort, except the gas pain. When they pump the gas into you, it finds its way out through your muscle fiber, and that hurts, but it's not unbearable.
Your surgeon is a good resource for that aspect of it, but your nutritionist or dietician is God. I can't speak intelligently about phases, so listen to them. But drink METAMUCIL regardless. Even if you're post-op, add it. And after you drink your little glass of fiber lax, follow it with a big glass of water, and keep drinking water. I can't underestimate the importance of water.