We have not added flag shell completion in the past, what is the reason for adding it for json?

We haven't, but --json has a static set of supported fields that users might need help spelling correctly, so shell completion is a great tool to help them out there. We can start adding completions for other flags as well— it's just that we'd have to hit the API to determine allowed values for most other flags, so that project will be slightly more complex.

Are we worried about it not working across all shells?

It will work across bash, zsh, fish, and PowerShell due to Cobra's recently improved completion support. When it doesn't, it's an issue that we should raise in Cobra's issue tracker.