I've been actively working on my language / compiler for a couple of years now (https://github.com/taolson/Admiran). My goal in its development was to learn in a deep sense how to efficiently compile a pure functional language down to low-level assembly-language, so I haven't used LLMs at all -- using them would negate the learning experience. Plus it's fun to puzzle through these things by yourself!

I actively use it for most programming, these days (command-line only), and use it every year during Advent of Code.