transitions in Bullseye: libcfitsio, libgsl, ...
There are references to libcfitsio[257] in lscsoft-external-cbc and lscsoft-external-periodic - Bullseye has passed a transition to libcfitsio8 in the meantime. IMHO it would be sufficient to use libcfitsio-dev which has the proper dependency on the distro-specific shlibs package, as was already done for lscsoft-lalsuite-dev.