The "Assigned To" column is not present in the "Tokens" OU in Active Directory Users and Computers (ADUC).
If you create the Defender OU and associated sub OUs using the standard "New | Organization Unit" option the "Assigned To" column will not be available. The Microsoft ADUC snap-in doesn't allow you to customize the column headings.
If you wish to restore the missing "Assigned To" column you can create a new "Token" OU by selecting "New | Defender Token Organization Unit" (with the Defender Console MMC installed). This will create a new Defender Token OU with the name of your choice and it should display the "Assigned To" column and the Key icon on the OU.
Once this Defender Token OU has been created your tokens can be moved to this new location.
Tokens can be stored in any location in Active Directory, Defender will automatically locate them assuming the Defender service account has the correct permissions of the user/token.
This is a (minor) side effect of creating the OUs manually, rather than having the Defender MMC installation routine create them automatically. This does not mean that the Defender schema modification failed in anyway.