Package: iproute2 Version: 5.4.0-1 Severity: serious iproute2 build-depends on iptables-dev, which is no longer built by the iptables source package. It is still present in unstable as a cruft package, but is completely gone from testing.
Before it's removal iptables-dev was a transitional dummy package depending on a bunch of other dev packages, please determine which of those dev packages your package needs and adjust your build-dependencies accordingly.