In practice
It has been replaced in Core Web Vitals by Interaction to Next Paint, which measures every interaction rather than only the first.
The fix is the same either way: break up long JavaScript tasks and defer what is not needed for the first screen.
Why it matters
A page that looks ready but ignores a tap feels broken, and visitors leave rather than wait.
Input delay is concentrated on mid-range phones, which is exactly where most traffic and least testing sit.