iPhone still has 90% battery but telling me it has degraded and should be serviced
Yes - there are three reasons I’ve seen the service battery be issued:


The voltage will drop too soon even though there is full charge capacity enough to be above 80% “beginning to lose capacity” line.
The Full Charge Capacity isn’t enough to get normal life out of the battery.
Charging anomalies - the battery needs to behave when charging as well as when discharging.


It’s most common to have the first two, but all three can spark a “check engine” light on the battery sensors and Apple’s diagnostic tools will have much more detailed telemetry and analysis once you get that service done. That runs without opening the device so you may or may not need a battery or other parts repaired.



Once you’ve had a scan and gotten a quote, I recommend you delay the replacement as long as you can if it’s safe to charge. Better to get all the life you can out of the existing chemicals and battery if you can still make a convenient appointment later. Be sure to get it serviced before any long trip or if you can’t get the use you require of course, but arbitrarily replacing at 80% would take a couple years off my batteries since the original iPhone. I rarely replace at that point as I generally have good service close at hand.