Most aerospace stuff is. The thing is, they have reams of very specific rules about how it's coded, how to verify that code, and how to verify the compiler of that code, and how to verify the code output from that compiler. It's not an easy process to replace, but its proven reliable just by all the commercial planes flying every day without falling out of the sky.

In theory, something like Rust could do the job instead, but they'd still have to verify the entire chain. Rust is for the rest of us to get something half as reliable as that while also being able to write more than two lines of code per day.