Image GenAI is diffusion-based, and I would say the image GenAI in Claude, Gemini and ChatGPT are all “in mainstream use”.

I've heard of attempts to use diffusion models to generate text or code as well, but my impression is that it just didn't yield the level of results necessary to dethrone a frontier transformer model.

If by mainstream you mean "ChatGPT" or "Claude", then no.

But also, no one really knows as they're closed.

If you include non-language models, yes.