“The most striking difference to the classic APT solver is that solver3 always keeps manually installed packages around, it never offers to remove them.”
APT 2.9.3 ships a backtracking solver behind a –solver 3.0 flag. The headline feature is that it will not delete packages you explicitly asked for. Autoremove now follows only the strongest dependency chain, so the gcc versions that pile up for years finally get cleaned out. The new selling point of a package manager is that it stops uninstalling your software.