What is a stale CI?
Stale CIs are CIs which have not been updated in x amount of time, as configured in the staleness rule for such CI type.
https://docs.servicenow.com/bundle/quebec-servicenow-platform/page/product/configuration-management/task/t_CreateCMDBHealthStaleRule.html
Do you need to take action on it?
A stale CI in itself is not a bad thing, it just means it is data in your CMDB which is not being used. You don't have to do anything with the stale CI, unless you have some internal process/procedure which must be followed.
How can you take action on it?
One way would be with remediation rules.
https://docs.servicenow.com/bundle/paris-servicenow-platform/page/product/configuration-management/task/t_CreateCMDBRemediationRule.html
Finally, how to reduce stale CI count?
As mentioned in one of my previous point, unless you need to for some reason you could just leave them. However, many companies use staleness to determine:
1. Do they need to delete CIs?
2. Why has such CI not been updated in so long? Is there an issue with imports/discoveries?
3. Do they need to be archived?
4. If no issues and you need to maintain the CI for auditing, you could review the staleness rules for the class and update them. This way the CI would not be marked stale for as long as you need to maintain the CI. However, this would be mixing up two different things, Staleness x Auditing. But, how to handle this will be dependent on your company's decision as far as what to do with stale CIs, if anything at all.
