Hello, was searching for a rear bumper for my Outback 2010 for a while and just purchased one online. I'm now realizing it's a 2015/16/17 bumper. How incompatible is the bumper?
I think it's 100% incompatible from generation 4 to generation 5 whether you're talking about the bumper skin or the bumper beam (you did not specify).
You can go to Subaru Parts Online from Subaru of America to look up the correct parts and see "what this fits" some parts span multiple car generations. Things like bumper beams and skins usually not.