Use this search to find unused dashboards:

| rest /servicesNS/-/-/data/ui/views splunk_server=* 
| search isDashboard=1 
| rename eai:acl.app as app 
| fields title app 
| join type=left title 
[| search index=_internal sourcetype=splunk_web_access host=* user=* 
| rex field=uri_path ".*/(?<title>[^/]*)$" 
| stats latest(_time) as Time latest(user) as user by title
] 
| where isnotnull(Time) 
| eval Now=now() 
| eval "Days since last accessed"=round((Now-Time)/86400,2) 
| sort - "Days since last accessed" 
| convert ctime(Time) 
| fields - Now

Admin Notes – Fantastic query! I modified the SPL slightly as I had an issue when I copied it to my two test environments.