Remove set_variable_write_status and updating migration guide

docs/user_guide/v4-migration.md

@@ -344,10 +344,10 @@ Remove the <code>chunks</code> argument from your code
 <hr class="migration-divider" />
 
 <div class="migration-bubble migration-change">
-<code>ParticleFile</code> does not have a <code>to_write</code> argument
+<code>ParticleFile</code> does not have a <code>to_write="once"</code> argument
 </div>
 <div class="migration-bubble migration-how">
-Remove the <code>to_write</code> argument from your code
+Either write all timesteps of a variable, or none
 </div>
 <hr class="migration-divider" />
 

src/parcels/_core/particleset.py

@@ -351,18 +351,6 @@ def _num_error_particles(self):
         """
         return int(np.sum(self._data["state"] != StatusCode.Success))
 
-    def set_variable_write_status(self, var, write_status):
-        """Method to set the write status of a Variable.
-
-        Parameters
-        ----------
-        var :
-            Name of the variable (string)
-        write_status :
-            Write status of the variable (True, False or 'once')
-        """
-        self._data[var].set_variable_write_status(write_status)
-
     def execute(
         self,
         kernels,

tests/test_particlefile.py

@@ -466,12 +466,6 @@ def test_particlefile_init_invalid(tmp_path):
         ParticleFile(path, outputdt=np.timedelta64(1, "s"))
 
 
-def test_pfile_write_custom_particle():
-    # Test the writing of a custom particle with variables that are to_write, some to_write once, and some not to_write
-    # ? This is more of an integration test... Should it be housed here?
-    ...
-
-
 def test_particlefile_readable_after_kernel_error(fieldset, tmp_parquet):
     """Parquet output file must be readable even if the kernel raises an error mid-execution (GH-2713)."""
 
@@ -489,32 +483,6 @@ def ErrorKernel(particles, fieldset):  # pragma: no cover
     assert len(df) >= 1  # at least the initial condition was written
 
 
-@pytest.mark.xfail(
-    reason="set_variable_write_status should be removed - with Particle writing defined on the particle level. GH2186"
-)
-def test_pfile_set_towrite_False(fieldset, tmp_parquet):
-    npart = 10
-    pset = ParticleSet(fieldset, pclass=Particle, x=np.linspace(0, 1, npart), y=0.5 * np.ones(npart))
-    pset.set_variable_write_status("z", False)
-    pset.set_variable_write_status("lat", False)
-    pfile = pset.ParticleFile(tmp_parquet, outputdt=1)
-
-    def Update_lon(particles, fieldset):  # pragma: no cover
-        particles.dx += 0.1
-
-    pset.execute(Update_lon, runtime=10, output_file=pfile)
-
-    ds = xr.open_zarr(tmp_parquet)
-    assert "t" in ds
-    assert "z" not in ds
-    assert "lat" not in ds
-    ds.close()
-
-    # For pytest purposes, we need to reset to original status
-    pset.set_variable_write_status("z", True)
-    pset.set_variable_write_status("lat", True)
-
-
 @pytest.mark.parametrize(
     "particle",
     [