> any language should come with a strict, non-configurable built-in formatter

Would that be on the language, or the IDEs that support it? Seems out of scope to the language itself, but maybe I'm misunderstanding.

I'd say a program or subcommand tha comes with the compiler/interpreter. Go has gofmt, zig has zig fmt. The compiler already parses the language so a formatter is only a few steps from that. And being part of the compiler means it's always up to date, there's less bikeshedding, all IDEs and text editors get formatter support easily.

The languave server, which in this age of LSP should come from the same project as the language itself.