I would love to be able to write Crystal for these types of platforms. I think one of the biggest reasons not much movement has happened on this front is that the memory requirements for Crystal are higher than many embedded platforms support. On top of that, it would take a lot of work hooking into the LLVM bits to output Arduino or ESP firmware.
Because of the memory requirements, I think it would almost be necessary to change Crystal so that the GC is optional (on by default, of course) before it could be ported to embedded platforms. That would be a huge undertaking into refactoring a majority if not the entire Crystal compiler. So if that was ever going to happen, it most certainly wouldn't be soon.
As far as the LLVM bits go, there has been some work on WASM LLVM support from Crystal, but again it takes a ton of work to get all the pieces working together. I think the best people to handle such a feature would be the core devs because they know the compiler the best. However, most of their time spent on Crystal is squashing bugs and getting features feature for a 1.0 release.
All that being said, if you really want this type of feature, it might be worth looking into making it happen yourself. If you do go that route, I'm sure a few people out there would help you.