Ceph

The feature was introduced for Dumpling. The only version that does not support it is Cuttlefish, and it will assert out when unable to decode the incremental version (as the encode/decode functions were protected by increasing the compat versions).

Considering that cuttlefish is currently considered really old, and that people tend to want to upgrade from cuttlefish instead of downgrading to cuttlefish, protecting us from dowgrades now should not be worth the effort of doing so and managing backports all the way back.

We must however avoid this sort of thing in the future. Even though mon protocol version was increased, and we took care of the encoding/decoding version barriers, the on-disk features should have been added back then.