PyQGIS: QgsVectorFileWriter.create() doesn't flush even after deleted

QgsVectorFileWriter.create() does not flush the output until QgsApplication.exitQgis() is called, because create() method uses QgsVectorFileWriter constructor method behind the scenes.Since it's deleted while exiting QGIS, it flushes the output.

Use newly added writeAsVectorFormatV2 method.

layer = iface.activeLayer()
file_path = "/Path/To/File.shp"

options = QgsVectorFileWriter.SaveVectorOptions()
options.driverName = "ESRI Shapefile"

QgsVectorFileWriter.writeAsVectorFormatV2(layer, file_path, QgsCoordinateTransformContext(), options)

It seems to be a bug that might be fixed in next release (3.16): https://github.com/qgis/QGIS/issues/37629

You can work around it by creating the file with QgsVectorFileWriter and then open the file and add the attributes and features again, somewhat like:

fields = src_layer.fields()

layer = iface.addVectorLayer(layer_path, layer_name , "ogr")
pr = layer.dataProvider()
pr.addAttributes(fields)
layer.updateFields()

fs = []
for src_f in src_layer.getFeatures():
    f = QgsFeature(src_f)
    f.setAttributes(src_f.attributes())
    f.setGeometry(some_kind_of_geom)
    fs.append(f)

pr.addFeatures(fs)