I feel like both soft shells and puffy vests are great layers, but less versatile than alternatives so probably not my choice for backpacking in most cases. Also, “soft shell” is such a broad term that I think its totally ambiguous what it actually means—some are highly breatheable, others breathe about like a garbage bag. You kind of have to specify which garment, theres that much variety.
Agree with others, outside of a specific narrow use case, if backpacking I would probably go with an active insulation layer, a hard-shell (a must-have to me backpacking, even if rarely used for rain) and a warm hooded puffy jacket. If I lived in a MUCH drier climate and had a good forecast and a short trip I might go softshell instead of a hardshell to be quieter and a bit more breatheable while covering any precip up to legit rain, but thats about it. Day trips I use a breatheable softshell a lot.