It's the capacity, not the soldering.

Probably not. The OS doesn’t care if RAM is soldered or socketed — it just sees memory. What will limit future upgrades is if you don’t have enough RAM for the next version’s requirements.

Soldered RAM means you can’t change it later. So if you buy a laptop with 8GB soldered, that’s your max forever. Future Windows or macOS releases might run, but they’ll chug if they need 16GB. The soldering isn’t the blocker — the cap is.

If you’re buying a soldered-RAM laptop, get at least 16GB now. You won’t regret it.