You can grant or remove the linger state permission from users and applications.
Default is usually that either any user can have any software lingering, or that any user can authenticate (as you'd do with e.g. sudo) to set this state. This would work via PolicyKit and elevated permissions, similar to the UA prompt on Windows.
You can also white- or blacklist individual applications and services from this :)
That's what I meant with "proper" API in contrast to old tmux/nohupd.
And as service management is the init system's task, it's clear this is something where you have to interface with the init system (as the init system is actually even supposed to reap any process reparented to it to reduce zombie processes).
