"Cut out" a tub's side???? Not if you want the tub to hold water!

If you are talking about one of the specialty expensive walk in tubs, they have their own set of problems. And the gasket that keeps them from leaking is on the list. As is the fact that you have to sit there in the tub until all of the water drains out so you can open the door and get out. These problems are solvable with some key add ons and regular maintenance, but the up front cost is still prohibitive for many.

Yes your correct, the tub becomes a shower only. It seems to be a alot less expensive than a complete tub to shower redo.

I've never installed one, but for the other readers, I'm pretty sure this is what the OP is referring too:

There are several different versions. Some like this, some with a door. I'd never recommend a door version though, as you're depending on a couple of tubes of caulk to hold back the flood waters. And yes, when properly installed, a couple of tubes of caulk will indeed hold back the flood waters. Until they don't and you get a flood. Which you will.

If this was for an elderly person, I'd keep the tub intact and instead just use a medical transfer seat to get in and out of the tub.