Unindexed Items in eDiscovery Content Search

Scenario: Sometimes, there is query from the customers that there is a noticeable size of unindexed items generated after the eDiscovery searches get completed. They need to know what items does Microsoft consider as unindexed items:

Here the details below:

Microsoft considers all of the below mentioned as unindexed items:

1. Encrypted documents in email 
2. Password-protected emails (a common scenario) like S/MIME encrypted emails.
3. Unsupported file types.
4. A file is encrypted with non-Microsoft technologies.
5. The file type is unrecognized or unsupported for indexing.
6. Messages have an attached file without a valid handler, such as image files; this is the most common cause of partially indexed email items.
7. The file type is supported for indexing but an indexing error occurred for a specific file.
8. Too many files attached to an email message.
9. A file attached to an email message is too large.

For more details and information, refer to the below MS article:

Partially indexed items in eDiscovery:

https://docs.microsoft.com/en-us/microsoft-365/compliance/partially-indexed-items-in-content-search?view=o365-worldwide 

Note:

Microsoft has removed the purge function and the purge function will never delete an unindexed item.

They also have removed the ability to exclude unindexed items from search due to legal reasons but you can exclude unindexed on export, but not on search anymore like you used to be able to.