Calling
s3cmd expire s3://bkt/ --expiry-days 1
Produces
<LifecycleConfiguration> <Rule> <Prefix></Prefix> <Status>Enabled</Status> <Expiration> <Days>1</Days> </Expiration> </Rule></LifecycleConfiguration>
Which uses obsolete field Rule/Prefix and it shall be replaced with Rule/Filter/Prefix
from doc:
Prefix identifying one or more objects to which the rule applies. This is no longer used; use Filter instead.

AWS still support the obsolete version, but others may not. The AWS examples only use the new version.