It's in the standard library for Python, Rust, Julia, and maybe some other languages. It's also widely used in those ecosystems (pyproject.toml, cargo.toml). I think it's fair to say it took off, even though YAML is also popular.

Rust uses it, and Rust seems pretty popular.

I know Alire, the Ada crate manager uses it too.

I use it for some personal projects. It's really nice!

Which is hilarious when you consider that the spec is that complex.

Toml is both great and terrible. I'm not a fan of how it handles some deeper arrays