The Squamish has a standard DWR finish that must be renewed periodically. The 'gossamera' fabric also has a very thin, air permeable PU coating on the inner surface which helps with water resistance, according to Arcteryx. This PU coating should be much longer lasting than the DWR, but it won't prevent the face fabric from wetting out if the DWR is shot.
Apparently the Patagonia Houdini has a lighter form of silicone encapsulation (like Epic), which makes it probably the longest lasting DWR of any windshirt.
The Squamish windshell is all but sold out in N. America, though they will reintroduce it as the Squamish Pullover in the spring.