Hi all 
After some years of use of non equal to 1 aspect ratio when unwrapping a mesh on an non square texture, yesterday i fell off my chair !!!
I unwrapped a mesh on a 4096x512 texture and the unwrap had an aspect ratio of 1 !!!
At first i thought this problem was for the worst a bug or at least a choice from developpers but now i know it’s a parameter !
lemme explain:
here’s what i use to have when unwrapping a mesh on non square texture:
Here we see clearly the unwrap is 4 times higher than it should be. 4:1 is my texture aspect ratio. I just have to select all UVs the S and 0.25 and it’s okay.
When using a square texture, UVs have a 1:1 aspect ratio.
now look at this:
Even if we don’t see clearly the UVs ( as the texture is tall ) we can clearly see they are unwrapped in a 1:1 aspect ratio !!!
WTF am i missing ???
where is that parameter that defines UVs aspect ratio on the texture ???
I bet this is a damn simple checkbox somewhere 
ah and… all my meshes have a scale of 1:1:1
Any idea is really welcome !
thanks and happy blending !
EDIT: i have to mention that the mesh of the second pic was first mapped on a square texture ! the i changed the texture to a 8:1 aspect ratio texture, and re-unwrapped it.
maybe it has something to do with UVs aspect ratio ? maybe Blender still takes the 1st texture aspect ratio ? i got no idea 
i’m not sure. Do you mean the one where you choose conformal/angle based, etc?