Maybe the port is not distributable, I've listed several situations in the parent comment where ports would be built from source, if none applies in your case you can ask on the macports-dev mailing list posting the output of port -v install [port].
> provided the maintainer policy allows it
We welcome any pull requests, the maintainer policy is for committers merging PRs.
> I am curious about new ports, though: what is the policy on these? Should we aim to "mirror" packages that homebrew/core adds?
Generally, any open-source software can be added to MacPorts. Feel free to add software Homebrew provides, but don't limit yourself to that :)
> Does MacPorts have any equivalent for Homebrew's Caskroom?
No, but you can use a custom ports tree (https://guide.macports.org/#development.local-repositories). In some cases (e.g. OpenJDK) we would repack binaries, but we prefer building ports from source (on our Buildbot or during install if binaries are not distributable).
We could discuss this on macports-dev. Creating a repository on GitHub and writing (un)install instructions is all we need to get started.
> https://trac.macports.org/ticket/57950
Our Trac tickets are somewhat neglected, please post on macports-dev if no one responds. If the change is simple, you can also make a pull request which is easier to review and merge.
> has there been any work done on making this migration between OS versions less jarring?
Yes, but the PR is stalled for now (lack of manpower again). https://github.com/macports/macports-base/pull/56
I haven't experienced these issues myself because I would always reinstall MacPorts after a major macOS upgrade.