However, there's no reliable way for as a user to tell if Steam will be optional for a particular game. I've tried checking pcgamingwiki but it's not always right.
Valve's own games can't be played without Steam. If Steam isn't running when you open Portal 2, Steam will be launched automatically. If you uninstall Steam, Portal 2 just won't start.
It would be one thing if Valve was setting an example that publishers were choosing to ignore, but that's not what's happening.
They might offer a small block of metadata indicating which parts of the API the change uses in its current incarnation. Would fit in nicely with the details about which platforms it can run on.
And it's usually not optional.