It's very possible other scenarios but SIGQUIT can cause same deadlock.
Looking at the systemd code, I see this in particular:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
 801 void sigkill_wait(pid_t pid) {
 802         assert(pid > 1);
 803 
 804         if (kill(pid, SIGKILL) > 0)
 805                 (void) wait_for_terminate(pid, NULL);
 806 }

 819 void sigterm_wait(pid_t pid) {
 820         assert(pid > 1);
 821 
 822         if (kill_and_sigcont(pid, SIGTERM) > 0)
 823                 (void) wait_for_terminate(pid, NULL);
 824 }
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

sigterm_wait() is called by some DBus thing:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
1067 static void bus_kill_exec(sd_bus *bus) {
1068         if (pid_is_valid(bus->busexec_pid) > 0) {
1069                 sigterm_wait(bus->busexec_pid);
1070                 bus->busexec_pid = 0;
1071         }
1072 }
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

I don't know how to reproduce this code path, but I guess that if PID 1 sends a TERM to that other process, which in turn starts dumping core (because it didn't like receiving the signal), then same deadlock will happen.

(In reply to Renaud Métrich from comment #1)
> I don't know how to reproduce this code path, but I guess that if PID 1
> sends a TERM to that other process, which in turn starts dumping core
> (because it didn't like receiving the signal), then same deadlock will
> happen.

This code path cannot be triggered in PID1 (or anywhere else in systemd codebase). It requires direct use of sd-bus API, like this:

int main(void) {
        sd_bus *bus;

        sd_bus_new(&bus);
        sd_bus_set_exec(bus, path, argv); // Sets bus->exec_path
        sd_bus_start(bus); // Starts the command in bus->exec_path and sets bus->busexec_pid accordingly
        sd_bus_close(bus); // Calls bus_kill_exec()
}

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