One of the andvantages of Snap packages over DEB ones is that the dependencies are contained in the package, hence a Snap should not break even if the system is updated.
Some old programs were meant to run on top of Qt3 and they have been removed from the repositories, because Ubuntu dropped support for it. I was thinking it might be possible to build a snap for such programs, but the easier way would be to install snapcraft on an old Ubuntu virtual machine and build the snap there.
So, what is the oldest Ubuntu that can run it? I'm not looking for a supported installation path or pre-built binaries. Just the practical limit, even if I were to build snapcraft from source.
I suspect you'll quickly run into issues trying to run snapcraft 2.x on anything older than xenial, as it has dependencies that are only available in xenial.
I suggest trying to build Qt3 from source as a snapcraft part.