Is iPhone battery replacement covered by the warranty?

If your battery health drops to 80% or so in the first year (the Limited Warranty) or while under the protection of AppleCare+, then yes. You may receive a free battery replacement.

iPhone Battery Repair & Replacement - Apple Support

Sorry about the duplicate, Bob!

Yes, if the Battery Health has dropped under 80% and the phone is still covered by the limited 1 year warranty.