Like I've stated multiple times, it'd be disabled when playing online like it always is.
To be fair, this is more a feature I'd like to see in general, mainly for if you want to jump out of the game, especially since you can't pause these games, but don't want to lose progress or wait for the whole thing to switch back on etc., because for example, if I shut my PS4 down while I'm in game on Dark Souls and then power it back up, I'm waiting like 5 minutes or so for the console to boot back up, the game to load, all the developer logos to go off, the game to connect online, then my save to load. Just hold options/the menu button to quit game, rather than going through menus, especially when you get games that make legitimately quitting them awkward. LIke, to quit some KH games you either need to find a save point and quit from there, do the all shoulder buttons plus start or whatever combination, or reset the console/quit application, and there was something I played on PC, an Assassin's Creed I think, where I had to basically quit the game 3 times before I could actually quit to desktop.
I've not played Dark Souls on Switch, but I assume putting your console to sleep and waking it up again while playing online will kick you to the title screen, but with an error message? That's basically all I'm suggesting, but a legitimate version of that which exits correctly.
I admit that I'm thinking more in terms of an online game here, just due to the fact you can't really leave them unless you quit out, as they're still live at all times, even if it's just AI in Demon's Souls case. Like, if I'm playing OW and there's a knock on the door, a phone call, a pet needs something etc, i'm just gonna quit out of the match, deal with it, and carry on, rather than shut the whole application or console down, and this being a simple shortcut, no matter how simple bringing up the menu, scrolling across to quit, and pressing confirm is, would benefit many titles and players.