Hi --  A common rule of thumb is to keep all CI's, but when they are no longer used/needed, set their status to something meaningful to you (uninstalled, removed, whatever...) so your reports/dashboards, etc. don't include this historical data.  Why keep it at all?  Because what if CI's are linked to incidents, problems, change records?  or historical or accounting data/reports over a period of time.

Keeping the CMDB healthy and reliable as a system of record is valuable but that doesnt mean deleting CI's that used to be used... just use the platform to 'mark' them as unused via some of the status fields, ownership fields, etc.

Hope this helps?