Winter '27 delivers a fix that support admins have requested since 2017: case milestones can now be deleted.
Where
On the CaseMilestone object, via standard data tools such as Data Loader or programmatically through Apex.
How
Outdated milestone records can be removed like records of other objects. Previously there was no supported way to clear them, so milestones piled up on cases indefinitely, even when a high-severity case was reclassified as minor and its old milestones no longer applied.
Why
In high-volume orgs, undeletable milestones quietly inflated storage costs and cluttered case records for years. Admins finally have a path to reclaim storage and keep entitlement data accurate instead of living with the bloat.