deleting the current key backup version is unspecified behaviour

do we roll back to the previous version? or behave as if there is no current version? or disallow doing that?

Locate the Matrix key-backup specification section that defines the current backup version and review how deletion is described. Determine which behavior should be specified when the current version is deleted, then document the chosen behavior and add any needed examples or conformance wording.

Written by the indexing model from the issue text.