/usr/bin/cal moved to its own package (ncal) in bsdmainutils upload
12.1.3. This is the version included in current stable and newer; but
it's after the version in oldstable (buster).
IOW, On buster, if you had installed bsdmainutils, you would get
/usr/bin/cal. bsdmainutils is Priority: important in buster but only
Priority: optional in stable onwards. That priority applies to all the
binary packages built from the source, including ncal.
Quoting the Debian FAQ:
https://www.debian.org/doc/manuals/debian-faq/pkg-basics.en.html#priority
If you do a default Debian installation all the packages of priority
Standard or higher will be installed in your system. If you select
pre-defined tasks you will get lower priority packages too.
So installing Buster, you would get /usr/bin/cal by default. Installing
anything newer, and you don't.
On upgrade from Buster, bsdmainutils will no longer provide
/usr/bin/cal. There's no dependency in place to automatically pull in
the ncal package, you have to do that yourself.