You should read Spivak's actual essay, 'Can the subaltern speak?' because it makes the case about how voices are excluded, even now, in our supposedly 'enlightened times'. I can only assume you haven't read it, from the way you ask this question.
In Postcolonial Theory subaltern means
'colonial populations who are socially, politically, and
geographically excluded from the hierarchy of power of an imperial
colony and from the metropolitan homeland of an empire' (see
Wikipedia).
That is, originating in a specific power and cultural dynamic. India had a bigger ship-building industry than the UK before the arrival of the British Empire, seeing them as all uneducated and landless is exactly the problem.
Spivak's point is not that there's a Stockholm-syndrome for the oppressed built in to their absorbing oppressor culture, although that is in the mix. It's about continuing incomprehension that sati, widow-burning, could be a free and volitional act of rebellion exactly against a primary excuse the British used for invasion. That is, they could speak, but the meaning they imparted to their actions could not transfer between cultures - silencing them.
Obviously changing power dynamics, new methodologies of imperialism, and of cultural colonialism, mean this is a shifting debate - and one it has to be said philosophy has been unusually and dynamically important in shaping. You might like to look at this discussion of Epistemic Justice for modern applications of the idea of subaltern: Need help with this paper on epistemic justice
A better example than Civil Rights I think, is Haiti and it's national religion being Voudou. There is still a kind of horror and confusion among former colonial powers about this different mode of religious discourse.
I would say that it is when, people are denied art and music and culture as modes of cultivating community self-awareness and dynamic change, that they become subaltern. When people are denied community-awareness and collective-creation-reform, often through external imposition of a cultural hegemony, they become subaltern just as much as when a colonial power invades and tells people how to live.