I don't have the answer, just leaving some comments for now. 1. "checks for the boost version it was compiled with" sounds like a useful thing for an upstream, but maybe less so for a distribution. This might be a similar issue to #1127799 "must depend on mold where it was used for building".
2. deal.ii's autopkgtests seem to struggle **during** transitions, see also #1115118 "autopkgtest regression on riscv64" which seemed to resolve itself once the correct combination was triggered, manually by me for riscv64, and automatically on other architectures. Maybe we should forget about smooth upgrades for deal.ii, and just add the Breaks. 3. deal.ii's autopkgtests only compile some test projects, maybe this is not a good autopkgtest and it should be dropped until there's a package that depends on deal.ii and can run better tests.
