um now uses the Kramdown gem to convert Markdown files into roff man pages.
On the one hand, this is great, because Pandoc was a heavy external dependency.
On the other hand, the Kramdown flavor of Markdown is not the same as the
Pandoc flavor.
If you have a lot of um pages written in Markdown and you are happy using
Pandoc, I encourage you not to upgrade to 4.0. You will have to revise your
um pages to make them work with Kramdown. um is an extrememly simple program
and is unlikely to change in the future, so you aren't missing anything. The
switch to Kramdown just makes um easier to package.
If you want to learn more about how Kramdown expects a Markdown man page to look, see this guide.
The old Pandoc format is documented in the Pandoc user's manual, available here.
If you really want to upgrade to 4.0 and migrate your Pandoc um pages, you can
take a look at the
changes
made to um's own man pages for an example of how to do that.
You might also want to look at these changes to the README.