/usr/include/osmium/index/ is in libosmium2-dev 2.6.0-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/osmium/index/bool_vector.hpp
/usr/include/osmium/index/detail/
/usr/include/osmium/index/detail/create_map_with_fd.hpp
/usr/include/osmium/index/detail/mmap_vector_anon.hpp
/usr/include/osmium/index/detail/mmap_vector_base.hpp
/usr/include/osmium/index/detail/mmap_vector_file.hpp
/usr/include/osmium/index/detail/tmpfile.hpp
/usr/include/osmium/index/detail/vector_map.hpp
/usr/include/osmium/index/detail/vector_multimap.hpp
/usr/include/osmium/index/index.hpp
/usr/include/osmium/index/map/
/usr/include/osmium/index/map.hpp
/usr/include/osmium/index/map/all.hpp
/usr/include/osmium/index/map/dense_file_array.hpp
/usr/include/osmium/index/map/dense_mem_array.hpp
/usr/include/osmium/index/map/dense_mmap_array.hpp
/usr/include/osmium/index/map/dummy.hpp
/usr/include/osmium/index/map/sparse_file_array.hpp
/usr/include/osmium/index/map/sparse_mem_array.hpp
/usr/include/osmium/index/map/sparse_mem_map.hpp
/usr/include/osmium/index/map/sparse_mem_table.hpp
/usr/include/osmium/index/map/sparse_mmap_array.hpp
/usr/include/osmium/index/multimap/
/usr/include/osmium/index/multimap.hpp
/usr/include/osmium/index/multimap/all.hpp
/usr/include/osmium/index/multimap/hybrid.hpp
/usr/include/osmium/index/multimap/sparse_file_array.hpp
/usr/include/osmium/index/multimap/sparse_mem_array.hpp
/usr/include/osmium/index/multimap/sparse_mem_multimap.hpp
/usr/include/osmium/index/multimap/sparse_mmap_array.hpp
/usr/include/osmium/index/node_locations_map.hpp