Replying to myself because a vouch wasn't enough to bring the post back from the dead. They were partially right and educated me. The downvotes were unnecessary. MS did start advising against dangerous deserializers 8yrs ago. They were only deprecated three years ago though, and only removed last year. Some of the remaining are only mostly safe and then only if you follow best practice. So it isn't a problem entirely of the past, but it has gotten a lot better.

Unless you are writing formal proofs nothing is completely safe, GC languages had found a sweet spot until increased concurrency started uncovering thread safety problems. Rust seems to have found a sweet spot that is usable despite the grumbling. It could probably be made a bit easier. The compiler already knows when something needs to be send or synch, and it could just do that invisibly, but that would lead people to code in a way that had lots of locking which is slow and generates deadlocks too often. This way the wordiness of shared mutable state steers you towards avoiding it except when a functional design pattern wouldn't be performant. If you have to use mutex a lot in rust stop fighting the borrow checker and listen to what it is saying.