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That is a very good question. Thank you for revisiting this! I remember getting tripped up by this in a unit test because the warning got hidden amongst a bunch of other warnings from the full test suite.

I tried the following with Python 3.11, NumPy 1.24, and Astropy 5.3.1:

>>> import numpy as np, astropy.units as u, warnings
>>> warnings.filterwarnings("always")

>>> float(5+5j)  # regular complex number
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: float() argument must be a string or a real number, not 'complex'

>>> float(np.complex128(5+5j))  # NumPy complex number
<stdin>:1: ComplexWarning: Casting complex values to real discards the imaginary part
5.0

>>> float(u.Quantity(5+5j, u.dimensionless_unscaled))  # Quantity from Python complex number
.../astropy/units/quantity.py:1350: ComplexWarning: Casting complex values to real discards the imaginary part
return float(self.to_value(dimensionless_unscaled))
5.0

>>> float(u.Quantity(np.complex128(5+5j), u.dimensionless_unscaled))  # Quantity from NumPy complex number
.../astropy/units/quantity.py:1350: ComplexWarning: Casting complex values to real discards the imaginary part
return float(self.to_value(dimensionless_unscaled))
5.0

Using float on Quantity objects now appears to give the same result with the ComplexWarning regardless of whether the Quantity was created from a complex or a numpy.complex128. Since there are warnings and exceptions in all of the above cases, I'm okay with removing this section.

Related to #10010, #10014, & numpy/numpy#13007.