Currently, there is no clarity on which custom fields are being tracked and display 'Last date used' column values, and which are not. 

We could get this information from the developers, but I think it's worth making this information public and available for everyone.

Because as part of the custom fields cleanup process, it's not on the surface why some custom fields show "Last date used" and which do not, which leads to confusion.

Here's the info shared by the developers:

When is “NOT_TRACKED” returned?

The following list of field types are “tracked”, which means all other custom fields that do not use one of these type return “NOT_TRACKED” in the REST API: https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-fields/#api-rest-api-3-field-search-get

com.atlassian.jira.plugin.system.customfieldtypes:multigrouppicker com.atlassian.jira.plugin.system.customfieldtypes:grouppicker com.atlassian.jira.plugin.system.customfieldtypes:labels com.atlassian.jira.plugin.system.customfieldtypes:multiuserpicker com.atlassian.jira.plugin.system.customfieldtypes:userpicker com.atlassian.jira.plugin.system.customfieldtypes:datepicker com.atlassian.jira.plugin.system.customfieldtypes:datetime com.atlassian.jira.plugin.system.customfieldtypes:select com.atlassian.jira.plugin.system.customfieldtypes:cascadingselect com.atlassian.jira.plugin.system.customfieldtypes:multiselect com.atlassian.jira.plugin.system.customfieldtypes:textfield com.atlassian.jira.plugin.system.customfieldtypes:textarea com.atlassian.jira.plugin.system.customfieldtypes:multicheckboxes com.atlassian.jira.plugin.system.customfieldtypes:float com.atlassian.jira.plugin.system.customfieldtypes:radiobuttons com.atlassian.jira.plugin.system.customfieldtypes:url com.atlassian.jira.plugin.system.customfieldtypes:multiversion com.atlassian.jira.plugin.system.customfieldtypes:version com.pyxis.greenhopper.jira:gh-sprint 
com.pyxis.greenhopper.jira:jsw-story-points 
com.pyxis.greenhopper.jira:gh-epic-color 
com.pyxis.greenhopper.jira:gh-epic-label

Putting everything into a table:

 

Scenario	Field type included in tracked list	Has a date last used value saved	Tracking status
System field	N/A	N/A	NOT_TRACKED
Custom field	no	N/A	NOT_TRACKED
Custom field	yes	no	NO_INFORMATION
Custom field	yes	yes	TRACKED

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Please, create a public document for all the customers to share this valuable information.