I'm wondering how they handle version skew and migration.

Fields have a lifecycle. When they're introduced, no clients or servers know about them. Clients and servers aren't restarted all at once. They won't have the same version of the types. Data will be written using one version of a type and read using a different version.

If you can guarantee all binaries, running programs, and data gets upgraded (no persistent data exists) then it might not be a problem. As soon as you have multiple organizations involved, guaranteeing all apps and servers sync to the latest version of a library, rebuild, and redeploy will be difficult.

Static type checking assumes no version skew. All the code in the binary got built with the same version of the library defining the types. It's a closed world.