When deleting a value from a Multi-Select Pick list, the value still needs to be kept in the system

In case a value of a Multi-Select Pick list is not longer valid we want to keep this value in the reports, even if you can no longer select this value for new fields.

If we delete the value in the settings, it's gone (and no longer shown in reports)
We would like to keep the value in the settings, but block people from still selecting it (as not longer valid anymore).

This functionality exist for a normal pick list, but not yet for a multi-select pick list.

Link to the documentation: https://success.clarizen.com/hc/en-us/articles/360000976574#h_529812572711518604562526

Technical explanation from support team: 
Since multi picklist is bit, and not a record in the DB- it is limited to 120, therefore we permanently delete the values when deleted, to leave a space for the customer to add more