They'll likely remain in the DB until such time that somebody can confirm, without doubt, that the radios / frequencies not only are no longer in use by the agencies but the agencies no longer have the capability of using those frequencies if they want to [eg: radios removed from vehicles/bases]. Otherwise, they do stay in the database as long as there is a chance that the agency could spring into action and use them as a backup or something.
And for us to even mark these as deprecated, we need to hear reports from people confirming "we haven't heard activity on such and such frequencies for weeks | months | years"