/usr/include/bullet/BulletSoftBody/ is in libbullet-dev 2.87+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/bullet/BulletSoftBody/btDefaultSoftBodySolver.h
/usr/include/bullet/BulletSoftBody/btSoftBody.h
/usr/include/bullet/BulletSoftBody/btSoftBodyConcaveCollisionAlgorithm.h
/usr/include/bullet/BulletSoftBody/btSoftBodyData.h
/usr/include/bullet/BulletSoftBody/btSoftBodyHelpers.h
/usr/include/bullet/BulletSoftBody/btSoftBodyInternals.h
/usr/include/bullet/BulletSoftBody/btSoftBodyRigidBodyCollisionConfiguration.h
/usr/include/bullet/BulletSoftBody/btSoftBodySolverVertexBuffer.h
/usr/include/bullet/BulletSoftBody/btSoftBodySolvers.h
/usr/include/bullet/BulletSoftBody/btSoftMultiBodyDynamicsWorld.h
/usr/include/bullet/BulletSoftBody/btSoftRigidCollisionAlgorithm.h
/usr/include/bullet/BulletSoftBody/btSoftRigidDynamicsWorld.h
/usr/include/bullet/BulletSoftBody/btSoftSoftCollisionAlgorithm.h
/usr/include/bullet/BulletSoftBody/btSparseSDF.h