Influence of I/O impedance in Butterworth passive filter design
That transfer function is written as Vout/Vin. Vin is a measurement after the effects of R_source, so of course R_source does not appear in the equation. 

If the transfer function was instead written as power_out/power_in, where power was that available from/to a matched load, the source impedance would indeed affect the transfer function, as it affects the level of Vin you'd measure at the input to the filter, due to the voltage divider action between power coming from the R_source impedance, and the impedance the filter presents.

Passive LC filters need at least one port to be resistive to provide damping, however making both ports resistive is the norm. This is because usually RF systems usually move signals around in a defined impedance system. It's also the case that the filter response is less affected by component value variations when it's doubly terminated.

You can design filters to have one port a short circuit, or an open circuit. This is very useful when building things like diplexers. The common port can, paradoxically, be thought of as a short circuit, as its voltage doesn't change with frequency. Design high and low pass filters with final series elements into a short circuit, and connect them. It's also very useful when building an interface to capacitive input load like Pockells cells. Design a filter into an open circuit, then absorb the load capacitance into the last shunt capacitor in the filter.

Even order Cheby filters have a dip at DC, which cannot be synthesised with equal terminations, which would give unity gain at DC.