I'd like to respectfully disagree here. IMHO, killing user processes after logout is totally reasonable and something I would expect to happen on desktop computers. On servers this might be a bit more difficult, but I like the idea of having to explicitly allow user processes to run outside of the session's scope. That keeps my server used by multiple people clean. I'd argue most sysadmins could simply deploy an alias to their servers for specific applications and be done with it.

edit: If you're already using a specific command to tell the application to stay alive (e.g. nohup), you can use the "new" systemd command as well. nohup never related to a "session" scope because that didn't really exist before systemd. A new command _does_ break historical behaviour, but thanks god we do that from time to time. That's how innovation works.