/usr/include/bullet/BulletDynamics/Featherstone/ is in libbullet-dev 2.87+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBody.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyConstraint.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyConstraintSolver.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyDynamicsWorld.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyFixedConstraint.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyGearConstraint.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyJointFeedback.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyJointLimitConstraint.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyJointMotor.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyLink.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyLinkCollider.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodyPoint2Point.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodySliderConstraint.h
/usr/include/bullet/BulletDynamics/Featherstone/btMultiBodySolverConstraint.h