A language which is not 1.0, and has repeatedly changed its IO implementation in a non-backwards-compatible way is certainly a courageous choice for production code.
So, I'm noodling around with writing a borrow checker for zig, and you don't get to appreciate this working with zig on a day to day level, but the internals of how the zig compiler works are AMAZING. Also, the io refactor will (I think) let me implement aliasing checking (alias xor mutable).
In my experience, migrating small-scale projects takes from minutes to single digit hours.
Standard library is changing. The core language semantics - not so much. You can update from std.ArrayListUnmanaged to std.array_list.Aligned with to greps.