By David Crowther
QGIS 3.4 and onwards allows a user to perform Interactive smoothing using the Simplify Feature tool.
Open the layer that you wish to smooth, make the layer editable and select the feature to smooth.
From the Advanced Digitising Toolbar choose the Simplify Feature Tool.
Then in the map window click on the polygon/polyline object to smooth.
Using the Tolerance settings you can choose to what degree you wish to simplify the feature, with the details showing how many features are being smoothed, the starting vertices and the vertices which will remain after the operation. When you are happy with the Tolerance chosen, press OK and the object(s) is simplified.
While the selected object(s) has now been smoothed with fewer vertices remaining, the overall shape of the original feature is still retained.