Comment # 3 on bug 991693 from Raymond Wooninck

No, it should be started automatically and there shouldn't be a need to place
it in the Autostart directory. However there were issues that it got started
and then directly died or got killed. The cause of it was never found, but it
seemed to be a kind of race of components, as that placing a sleep 1 in the
plasmashell.desktop file (in the exec= line) helped resolving this particular
issue.

With later versions this got somehow resolved, but unfortunately we can not
provide plasma updates anymore as that this would also require higher versions
of system libraries. 

With Leap 42.2 this issue should no longer be there.