it turns out ModemManager is started by (everyone's favorite) dbus.

the directory /usr/share/dbus-1/system-services contains systemd .service files.
at init-time dbus scans this directory and starts all the listed services.

i did not find documentation about the function of this directory, so it does
appear to just be baked-in to debian. this setup must be documented somewhere
but i did not find out where.

to disable ModemManager just rename the appropriate file so that it get ignored.

so, rename /usr/share/dbus-1/system-services/org.freedesktop.ModemManager1.service
to /usr/share/dbus-1/system-services/org.freedesktop.ModemManager1.service.disabled

the only catch is that if dbus/ModemManager every get updated this change probably
gets stomped-on and needs to be redone. but, since an update is very unlikely until
devuan excalibur rolls around, not too bad.

man service

SERVICE(8)                  System Manager's Manual                 SERVICE(8)

NAME
       service - run a System V init script

SYNOPSIS
       service SCRIPT COMMAND [OPTIONS]

       service --status-all

       service --help | -h | --version

For example

there must be something somewhere that instructs init to start this process,
i just cannot find it. i guess purging packages is the usual way in devuan?

swanson, thanks for the clue. sysv-rc-conf is very helpful. the command-line
dump gives a good, condensed view of init-time.

I don't know if it's running (how do you test it to find out?), but I did see Plasma-nm (network manager) depends on some other packages with the modemmanager in the name.

ymmv. All the best

ModemManager is not a 'service' , is not an XDG 'autostart', is not a KDE 'autostart'. it seems to
be baked-in. on my kubuntu box systemd has ModemManager 'masked' so that it never starts.

in devuan how do you stop/disable ModemManager?