$ cal
originally had highlighting on.

Now after installing this package, it is off.

However the man page makes it seem like maybe we aren't on a terminal.
But tty(1) says we are.

Therefore there should be a --force-highlighting-on to test.

So it turns out one needs
$ ncal -C
or
$ ncal -b
to do what
$ cal
used to do.
Not even
$ cal -C
will help.

By the way the man page makes it seem like we can type
$ cal -b
but that is an error.
Because the man page assumes we typed ncal, not cal,
even though both are in the SYNOPSIS.

Colour highlighting was intentionally removed when ncal is
invoked as cal, as per
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=904839
cal being there only for historical purposes to give the old
style output and with the same API (see also
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=867995)

I agree the man page could be improved to make it clear
highlighting is not enabled when invoked as cal or that ncal
should be the preferred command.

When invoked as ncal, the current day is highlighted, and you
can use ncal -C to get the same layout as with cal and
highlighting.

See also GNU cal (gcal) which highlights the current day and
has many (insanely) more features (the one I use personally is
gcal .. or gcal .+ to get a three month calendar; similar to
ncal -bB1 -A1 and ncal -bA2 respectively).

Its output is similar to that of ncal -b with Monday first as
is more natural in most parts of the world.