This package either build-depends on libboost-system-dev or depends on it.

The boost system library was removed upstream in 1.89, and is empty since 1.69.
If this library is referenced in the build system, the library can just be
removed from the build system, and the package should continue to build
for any boost version newer than 1.69, including boost1.83 in Debian.

There is no need to wait until boost 1.90 becomes the default, the issue
can be addressed in unstable. Issues to check:

  - Remove libboost-system-dev in Depends and Build-Depends.

  - Adjust autopkg tests and test dependencies if needed.

  - Scan the build system if the library is used, and remove it
    from the build system. https://www.boost.org/releases/1.89.0/
    has a short paragraph how to do that for cmake.

boost1.90 is now available in unstable, and a boost-defaults
defaulting to 1.90 is available in experimental. Please also check
that the package builds with boost 1.90.

control: severity -1 normal

not really:
https://packages.debian.org/sid/amd64/libboost-system1.83.0/filelist
https://packages.debian.org/sid/amd64/libboost-system1.83-dev/filelist

these are not empty packages.

For now amule will stick with boost 1.83, and this can get RC once
that's removed and 1.90 becomes the default. Upstream is in the
process of migrating to cmake, but that work has not
completed/released yet.