Just is a command runner that reads user-authored files, that are similar to makefiles, and runs the commands that those files contain. Because those commands are shell script snippets that expect to have the same permissions as the user, and see the same filesystem as the user, just requires classic confinement.
As a build tool which needs to execute arbitrary external commands, just meets the requirements for classic confinement. just also fits within one of the supported categories for classic confinement (as per Process for reviewing classic confinement snaps) which is compilers etc.
As such, the requirements for classic confinement are understood. @advocacy could you please perform publisher vetting? Thanks.