rounded corners On Lightroom Classic

"Rounded Corners"? Are you referring to the Vignette tool in the Effects panel? If so, that should already be available in Lightroom Mobile on the iPhone.

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.