Here we took an example as assignment group field which references the group table.
Steps to Reproduce:
Got to dictionary to 'task' table and column name 'asssingment_group'
Change the max length to 80 in the list
update max_length from 32 to 80 -- Update not applied max_length shows 32
Assignment group is Reference type field referenced to sys_user_group table.
Can see info message - Maximum length cannot be increased on a Reference type field. Column 'assignment_group' in table 'task' not modified.
The info message and the update operation are not happening due to the below business rule.
OOB Business rule does not allow to update the Reference field max length to more than 32 characters
Dictionary reference length - https://instancename.service-now.com/nav_to.do?uri=sys_script.do?sys_id=496a9259ff422000718b1c57f27f9d1c
A reference field always stores a sys_id that is 32 characters in length. The actual field that this is stored in is hidden when you look at a form but gets populated with the sys_id of the referenced record when you populate a valid value in the field you see. The field you actually type into is just for the display value of the reference field and the max length of 32 doesn't apply to the display value.
So reference field display value of the group name can use more than 32 characters, reference fields consider only 32 character sysID for referenced value but not its display value.
This is the reason the above business rule doesn't allow to modify the max length of the Reference field from 32. This is the expected behavior of the platform