Not traceless. After you've played a cart, an icon is left on your home screen (with the symbol that shows that you need the cart inserted to play). There is a footprint left behind.
You might say that it doesn't matter if it's your own backup of your own cart, but get real for a second. We've already been through all of this with the 3DS, even to the point of injecting a cert from a self-owned/dumped game, into an illegit .3ds dump. What you get is mistakes where a game is shared, then 2 people end up online using the same certs at the same time. Then the cart itself gets banned. And let me just roll back to the fact that the extra checks since 9.x prevent it from working anyway.
You can play your "backups on the go" offline in emummc. Emummc is the only truly traceless method.
Yeah, there is the icon and so on, but its no different to just using the gamecard in terms of traces, so its not really a trace showing you used a pirated or backed up copy.
Emummc makes things much more complicated and if you treat it as the safe way to do everything you want increases the chance of user errors.
Especially syncing savegames is annoying when using both emunand and sysnand. Like I mentioned, its not a big problem for pirating people, since they will never go online with it, but for people owning the games, and wanting to go online, needing to move savegame from emunand to sysnand to go online and then back to play it offline again is just so much work, it defeats the point of the backups.
About the sharing part, that's the problem of the person managing/sharing it, never had that problem on the 3ds, and never had that problem on the switch.
And injecting certs from other games into pirated switch games is something that's not even possible.
And if someone finds a way to circumvent/reimplement the challange answer Generation for the game cards, I'm pretty sure it will also only work with data dumped from that specific gamecard.
Also it's not like I just talk about it, I'm regularly checking if developments in the switch community made it easy enough for me to implement xci loading myself, but since I don't really have any reverse engineering experience, I will not be able to implement it until fs got reimplemented.