Can we replace the iPhone 8 battery?

From:

iPhone Battery Replacement - Official Apple Support

iPhone 8, iPhone 8 Plus, In warranty: 0$ Out of warranty: 49

These prices apply only to battery repairs made by Apple. Pricing offered by Apple Authorized Service Providers may vary. We'll add a $ 6.95 shipping fee if your repair requires shipping and isn't covered under warranty or AppleCare+. All fees are in USD and exclude local tax.

Replacing a battery under warranty is for very specific reasons, including premature failure or falling below Apple's nominal rating of 500 cycles to 80% battery health. But that would require it being under 500 cycles. Few meet that criteria, especially since it rarely dips below 80% in a one year warranty period. But it might be more likely under AppleCare's extended period.

However, most replacements happen out of warranty and are for Apple's fee or whatever an Apple Authorized Service Provider chooses to charge for the service. This can vary by country and by which AASP (if that's the option chosen). I've read about some countries where there as a dominant AASP that charged a mandatory diagnostic fee on top of the battery service price.