I found some old request about kontact (one is still unconfirmed, lol), but e.g., for kmail i still can't find any way to start it hidden. Basket has option to start hidden. And there are more such app (amarok - i enabled its startup to begin music on login).

Currently i see one partial workaround - add rules to kwin to minimize windows when they appear.

My proposal is to add option like --start-hidden to kde options (listed with --help-kde). And when some application is added autostart - this option could be implied by startup system, also it could be added by applications itself (if it has such option enabled in settings), or there could be separate desktop item for startup.

Reproducible: Always