arcpy.GetParameterAsText not passing arguments to script?

The code that you have presented is for a Python Toolbox (*.pyt) for which you need to use the Parameter class.

GetParameter() and GetParameterAsText() are only used with Python Script tools in Standard Toolboxes (*.tbx).

Using the answer from @PolyGeo my corrected code is as follows:

import arcpy


class Toolbox(object):
    def __init__(self):
        """Define the toolbox (the name of the toolbox is the name of the
        .pyt file)."""
        self.label = "Toolbox"
        self.alias = ""

        # List of tool classes associated with this toolbox
        self.tools = [Tool]


class Tool(object):
    def __init__(self):
        """Define the tool (tool name is the name of the class)."""
        self.label = "Test_tool"
        self.description = ""
        self.canRunInBackground = False

    def getParameterInfo(self):
        """Define parameter definitions"""
        param0 = arcpy.Parameter(
            displayName="Input workspace",
            name="workspace",
            datatype="DEWorkspace",
            parameterType="Required",
            direction="Input")
        param1 = arcpy.Parameter(
            displayName="Input classified raster",
            name="input_raster",
            datatype="GPRasterLayer",
            parameterType="Required",
            direction="Input")
        param2 = arcpy.Parameter(
            displayName="Input features",
            name="input_features",
            datatype="GPFeatureLayer",
            parameterType="Required",
            direction="Input")


        params = [param0, param1, param2]

        return params

    def execute(self, parameters, messages):
        """The source code of the tool."""

        # Define some paths/variables
        outWorkspace = parameters[0].valueAsText
        arcpy.env.workspace = outWorkspace
        output_location = parameters[0].valueAsText
        input_raster = parameters[1].valueAsText
        input_features = parameters[2].valueAsText

        output_features = output_location + "\\projected.shp"

        out_coordinate_system = arcpy.Describe(input_raster).spatialReference
        proj_grid = arcpy.Project_management(input_features, output_features, out_coordinate_system)

        return