Exposing user's internet speed can be very helpful to making a website that's adaptive to their network environment (e.g. loading smaller images on 3g).
Are there plans to implement Network Information API in Safari?
That API would be implemented in the WebKit project. While we don't discuss future plans, the WebKit Feature Status page can often provide some indication: https://webkit.org/status/. The Network Information API is not currently listed there, but stay tuned there for updates.