Currently (as per Redmine 3.0.0) it is possible to define custom fields that have "List" format. However there are circumstances when this does not work well:

Suppose a "Department" custom field defined as list ("Engineering", "QA", ...) assigned to users. When a list item has to be corrected, the change is not propagated. Each user entry holds the old value, not the item ID.¶

When a same list needs to be reused in another custom field (example "Customer" field can be part of Project and Issue object), too duplicate lists have to be defined.¶

The proposal is to allow custom enumerations and use them as one possible format for custom fields.

This feature #9235 request seems similar, but not sure.