"... and, as evidenced by the huge amount of work the debian folk are putting into repeatable builds, not a trivial one; certainly not one that dpkg has already

"... and, as evidenced by the huge amount of work the debian folk are putting into repeatable builds, not a trivial one; certainly not one that dpkg has already solved." The reproducible builds project in Debian is about reproducing the build and getting every single bit the same. I think that's the primary reason why it's hard - there are various innocent sources of non-determinism, e.g. dictionary ordering in Python (pre 3.6), time stamps, build environment details, etc. If you can settle for just getting a specific repeatable set of versions, I think Debian solved that ages ago, it's just that they don't keep old versions lingering around forever. By the way, if you take a step back and squint, I wouldn't be so sure OS level package managers really are that different from what what you call language package managers.