How to tell if my iPhone battery health is normal?

Batteries will degrade and will fail, inevitably and inexorably.

When your battery health drops below 80%, you’re headed for a replacement battery, or possibly for a replacement iPhone and that depending on how old your iPhone is by then, and what sort of shape it’s in otherwise.

The battery health will drop over time, but not necessarily in any particularly predictable timeframe. A mistreated battery can drop the health pretty fast when an iPhone is exposed to high temperatures and the battery badly overheated, for instance.

If you want your battery to last longer, consider enabling the optimized charging setting, and get on a consistent schedule to charge your iPhone.

And remember to use your iPhone for what you bought it for, which probably wasn’t as an inherently imprecise measurement of a battery.

Because it's informative.

It doesn't mean there is anything wrong, it's simply telling you that batteries don't last forever.

People apparently need to be told these things now in order to avoid lawsuits. Right up there with "caution, coffee is hot" and "do not iron clothes while wearing".