What's the Unix command-line symbol for "the PID of the last suspended process"?

You might be looking for the $! variable (bash manual, section Special Parameters).

However, you don't need the PID – the built-in kill command also accepts job identifiers, such as %2, which are shown when you press Ctrl-Z or type jobs. You can use %, %+ or %% to refer to the latest job. (Other possibilities are in bash manual, section Job Control.)

>>>
[4]+  Stopped                 python
$ kill %4

jobs -p %

It shows PID of last suspended job (after pressing Ctrl+z).