Comment 2 for bug 659328

The issue can be solved in two ways:

The milestone vocabularies that provide the milestones for search and bug targeting can exclude obsolete series and deactivated project. This may take longer to build the data, but the data is backward compatible with the state of the data.

or

Changing the status of a series or project sets all the milestones to inactive. This requires that the historic date be updated. This is also an unforgiving behaviour. Changing to the wrong status means there is a lot of work to correct the mistake. While it is obvious that an obsolete or deactivated project has no active milestones, it is not clear which milestones will need to be mad active.