You can easily add a new function to get array of z-values from line vertices.

1. Open Function Editor tab in Field Calculator (or Expression) window. Create a new function by clicking plus icon. Enter a file name, e.g. z_array_from_line.

2. Copy/Paste the script. Then, click Save and Load Functions (or related button) to register the function. It's added 'Custom' group.

    from qgis.core import *
    from qgis.gui import *
   
    @qgsfunction(args=0, group='Custom')
    def get_z_array_from_line_vertices(value1, feature, parent):
        z_array = [p.z() for p in feature.geometry().vertices()]
        return z_array
   

3. Now, you can use get_z_array_from_line_vertices() function in expressions.

The function gives you z-values in the same order as the vertices.

You can easily add a new function to get array of z-values from line vertices.

1. Open Function Editor tab in Field Calculator (or Expression) window. Create a new function by clicking plus icon. Enter a file name, e.g. z_array_from_line.

2. Copy/Paste the script. Then, click Save and Load Functions (or related button) to register the function. It's added 'Custom' group.

    from qgis.core import *
    from qgis.gui import *
   
    @qgsfunction(args=0, group='Custom')
    def get_z_array_from_line_vertices(value1, feature, parent):
        z_array = [p.z() for p in feature.geometry().vertices()]
        return z_array
   

3. Now, you can use get_z_array_from_line_vertices() function in expressions.

The function gives you z-values in the same order as the vertices.

EDIT: But max() function in Expression is expecting set of values like max(1,2,3), not list or array like max([1,2,3]). If you need maximum value you can add python max() which excepts list to the definition of the function or save as a new one.

from qgis.core import *
from qgis.gui import *

@qgsfunction(args=0, group='Custom')
def max_z_value_from_line_vertices(value, feature, parent):
    z_array = [p.x() for p in feature.geometry().vertices()]
    return max(z_array)