Moving a geometry using PyQGIS3

To change QgsFeature geometry in QGIS 3 you have to call setGeometry() explicitly.

geom = feat.geometry()
geom.translate(100, 100)
feat.setGeometry(geom)

Explanation:

In QGIS 2 geometry() returns a pointer, so it can be modified in place.

QgsGeometry* QgsFeature::geometry() //QGIS 2

In QGIS 3 geometry() returns value instead of a pointer. Additionally it is const now, so it is not allowed to modify the QgsFeature.

QgsGeometry QgsFeature::geometry() const //QGIS 3

(see https://qgis.org/api/2.18/classQgsFeature.html#ab0a934a1b173ce5ad8d13363c20ef3c8)

In your examples the manipulated geometry is not written back to data source. The Python Cookbook for PyQGIS offers two possibilities:

(1) using te methods changeAttributeValues(), changeGeometryValues() of the dataProvider (2) do that within an editing buffer, to have the possibility to explicte save in an commit and so on.

a solution for (1) based on your script:

layer = iface.activeLayer()

for feat in layer.getFeatures():
    geom = feat.geometry()
    print(geom.asWkt())
    success  = geom.translate(100, 100) # translation

    fid = feat.id()
    print('id:',fid)
    layer.dataProvider().changeGeometryValues({ fid : geom })

# reread geoms to see if they changed
for feat in layer.getFeatures():
    geom = feat.geometry()
    print(geom.asWkt())