Last user update for a table
Unless and until you try out some more observations and results of same, nothing can assure you 100 %. Although can assure you 99 % but rest you have to test.

Below query can give you what you said for dmv sys.dm_db_index_usage_stats as last modified date since restart on the usage of table.

SELECT last_user_update
FROM   sys.dm_db_index_usage_stats us
       JOIN sys.tables t
         ON t.object_id = us.object_id
WHERE  database_id = db_id()
       AND t.object_id = object_id('dbo.YourTable')


Also, below can give you an idea on same. 
Result might be same on last usage date, but depends what you are looking for.
You need to understand the nature of that table as well depending upon the data out there.

SELECT
        [name]
       ,create_date
       ,modify_date
FROM
        sys.views
WHERE
        modify_date &gt; DATEADD(day,-30,GETDATE())


In addition you will need trigger to observer what and when was it changed to analyse further.

Furthermore, if you have the default trace luckily out there , you might get some help that might get you close to what you are aiming for:
Read Collecting the Information in the Default Trace