Rounded corners never went away, because there has never been a feature named “Rounded Corners” in the Develop module.
If you have seen rounded corners created in the Develop module, it was probably a clever hack using the Post-Crop Vignetting feature in a certain way. And that was never removed, it’s still in the application today…see the demo below in Lightroom Classic 12.1. In short, set all the Post-Crop Vignetting sliders to minimum. If you want a white border, set Amount to maximum; and if you want a thinner border adjust Midpoint.
The same Post-Crop Vignetting feature is in the phone and tablet versions of Lightroom, under the Effects panel, so you can create those rounded corners anywhere.