Writing out PointM shapefiles from QGIS
You can use "Processing Toolbox > Vector Geometry > Drop M/Z Values" tool.
As workaround, you could use a virtual layer
, specifying a query similar to the following:
SELECT id, MakePointM(x, y, attribute) AS geometry FROM PointZM_layer;
And then export the virtual layer
as a shapefile.
Please note: I didn't have the chance to test this! :)
Reference: https://docs.qgis.org/2.14/en/docs/user_manual/working_with_vector/virtual_layers.html