9to5Mac:

Apple has extended its service coverage for Apple Watch (1st gen) models experiencing expanded/swollen battery problems to three years, covering customers for service for an additional two years beyond the complimentary 1-year Limited Warranty that it provides with the device.

The issue with swollen and expanded batteries on first-gen Apple Watch models has been documented online by users experiencing the problem— here on Apple support forums, for example, and here on Reddit— but appears to not be widespread or something that has made mainstream media headlines.

Good to know.