can you run a query against your database?
Jacky_Mahadab helped me build the following one, to find all the deployment plans that still use the process tag, you need to archive them all before you're able to delete the tag
select
rcdt.name as TemplateName
,rcrc.ID as PlanID
,rcrc.name as PlanName
,rcrc.build as PlanVersion
,c.PROCESS_NAME as ProcessName
,s.TAG_NAME as ProcessTag
from
rc_release_candidate as rcrc
inner join rc_rc_module rcrcm
on rcrcm.release_candidate_id=rcrc.id
inner join rc_basic_modules rcbm
on rcbm.id=rcrcm.id
inner join published_process s
on s.ID=rcbm.published_process
inner join process_container c
on c.ID=s.ID
inner join rc_deploy_template rcdt
on rcrc.deployment_template = rcdt.id
where c.PROCESS_NAME like 'PROCESSNAME' AND s.TAG_NAME like 'TAG'
Order by TemplateName,PlanID Desc
Make sure to replace "PROCESSNAME" and "TAG" in the "where" part