Following a suggestion by sthibault, I added texlive-plain-generic as build dep for doxygen-latex. With that change, I did a partial ratt job, focusing on the 9 build-dependencies of doxygen mentioned above. Details here: https://salsa.debian.org/paolog-guest/doxygen/wikis/ratt3
The new version fixed hwloc and starpu. For the remaining 7, they also fail without doxygen 1.8.15, so the FTBFS is "maybe unrelated to new changes". I have just launched a new full ratt job: https://salsa.debian.org/paolog-guest/doxygen/wikis/ratt4 P.