Apply a radius to sharp corners of a path. The extension is written for inkscape 1.0.1 (but also contains a backport for 0.92.4).
Usage:
The screenshot demonstrates the usage.
In the upper half, a red path with 4 vertices is shown. We select two vertices: the top left and bottom left corner (shown in blue).
Run "Rounded Corners" from "Extensions" -> "Modify Path".
The result is the path below. (Viewed here in edit mode again to show all the new vertices.)