If you guys want to see,
here's a map of the PA population change by municipality from 2010-2020.
In the western part of the state, high-growth is mostly restricted to a fringe of areas to the Northwest and Southwest of Pittsburgh. The northwest growth is at this point mostly in Butler County (since the North Hills suburbs are mostly built out) while the southwest growth has spread into Washington County. All of these areas are close to 79, which makes driving commutes fairly easy.
Outer Butler County is indeed shrinking, other than the township surrounding Slippery Rock. Arguably the decline isn't quite as bad as in some of the other exurban counties.
You can also see here that the exurban growth spilling into Westmoreland is much slower growth, which is why that county is in much worse shape. The basic reason is shitty commutes, as 376 is perpetually congested, largely due to being routed through an undersized former rail tunnel in the city. This is part of what pushed suburbanization in Pittsburgh to the north of the city, even though if you turn back to the mid 20th century the eastern suburbs were by far the most prominent and the North Hills was very underdeveloped.