Looks like DUs might be delivered partially over multiple versions of C# starting with Union classes based on a design meeting the team had the other day.
Click the image for a larger view in a new tab …

Looks like progress anyhow, and the phased approach means we’ll likely get some DU capabilities sooner than if MS tried to deliver all DU types in one version of C#.
No timelines yet, perhaps too soon for this Novembers C# 14 release but who knows, they do mention that Union classes seem pretty straight forward from an implementation view but this could be relative to other DU types not in general 🤷🏻♂️.
https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-01-13.md