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I mean in the context of users providing support for commercial products without being paid for it.

Stack Overflow is a general Q&A site, not a support forum for a commercial product. The context is very different. Also, while I'm at it, it has its own flaws, but people providing trite and useless answers isn't one, they get downvoted to oblivion.

Stackoverflow isn't company support, it's programmers supporting other programmers.

Which companies use stack overflow as their one and only customer support channel?

Presumably the company would rather have something like Stack Overflow, but with total control over the moderation.

I think Stack Overflow used to sell this as a service. Don't know if anyone used it.