Remove z-value from XYZ point in sf

Question

I have a sfc-object consisting of just points. Unfortunately, when I read in the shapefile, it created points of type XYZ. Now I removed some points and can't figure out how to export the remaining points as shapefile. I get the error:

GDAL Error 6: Geometry type of `3D Point' not supported in shapefiles

As I don't really need the third dimension So I thought I'd just remove it. But I don't know how. This is how the sfc-object looks like at the moment.

First 5 geometries:
POINT Z (397864 6302484 0)
POINT Z (386440.2 6299046 0)
POINT Z (390490.3 6296620 0)
POINT Z (391614.7 6293960 0)
POINT Z (377803.1 6276244 0)

Answer 1

Using the data from ?st_zm, here's a thing with Z and M coordinates:

> a
Simple feature collection with 2 features and 1 field
geometry type:  LINESTRING
dimension:      **XYZM**
bbox:           xmin: 1 ymin: 3 xmax: 8 ymax: 16
epsg (SRID):    NA
proj4string:    NA
  a                           geom
1 1 LINESTRING ZM (1 9 17 25, 2...
2 2 LINESTRING ZM (1 3 5 7, 2 4...

feed it into st_zm and bye bye Z and M:

> st_zm(a)
Simple feature collection with 2 features and 1 field
geometry type:  LINESTRING
dimension:      **XY**
bbox:           xmin: 1 ymin: 3 xmax: 8 ymax: 16
epsg (SRID):    NA
proj4string:    NA
  a                           geom
1 1 LINESTRING (1 9, 2 10, 3 11...
2 2          LINESTRING (1 3, 2 4)