Although starting and ending may be assumed from the text above it might be better to use a more verbose sentence for clarity; 

i.e. `The version number denoting starting which version the associated element is deprecated MAY be omitted. In this case the version number denoting the version in which the associated element will be removed MUST be preceded by a colon.`

As can be seen in the text above the terms MAY and MUST will also make the business rules more obvious, as intended.