As of November 2024, AWS introduced support for appending data to objects stored in Amazon S3 Express One Zone buckets.

• Documentation
• Release Note - Nov 21, 2024

Example in JavaScript AWS SDK v3

s3.send(
  new PutObjectCommand({
    Bucket: 'bucket',
    Key: 'fileKey',
    Body: 'some body',
    WriteOffsetBytes: 123,
  })
)