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It's primarily used by people who tend to sit on the cutting edge e.g. startups and developers who follow the latest tools. It's not well worn enough to be adopted by slower enterprise environments. Bun is well known within web development but if you don't work in the space and don't keep up to date with modern tooling it's unlikely you would have awareness of it.

I'd say the most prominent user (and the reason why Anthropic acquired Bun) is Claude Code

To my limited knowledge, "serious" production systems most likely use Node.js instead of any alternatives, and I don't see any movement towards adopting Bun.

notably anthropic on a multibillion revenue product