Description of problem: The RELEASE-NOTES and the package documentation says that both the old pthreads libraries and header (include) files are in the linuxthreads-devel package. OK, the libraries are there but the header files are in glibc-headers
That's on purpose, as on bi-arch arches only one set of headers is needed (on x86_64 the x86_64 one) while libraries are needed for both arches.
Plus there is no linuxthreads* in rawhide anymore...