In the zip file I downloaded, bullet won't build in my project with the soft body stuff because btSoftBody::CreateFromConvexHull is declared and used [at the end of btSoftBodyHelpers::CreateEllipsoid] but never actually defined anywhere in src/. So it won't link if I just drop the library part of bullet, including SoftBody, into my code [which is how I currently use bullet]. By removing SoftBody/ from my project, I didn't suffer this linking error, but then I also don't get access to the SoftBody stuff.
It's actually defined in Demos/SoftDemo/SoftDemo.cpp, but it depends on ConvexHull. Obviously it's there because ConvexHull is part of Extras, and you don't want anything in src/ depending on anything in Extras/... except you're depending on something in Demos/ instead, probably even worse
I would suggest that [assuming it's mature enough] ConvexHull just gets added to the main src/ directory and move the definition of btSoftBody::CreateFromConvexHull into btSoftBody.cpp, but I assume there's a reason it's still in Extras/ instead of in the main distribution, so, I dunno.