In DB2 V8 there is no way of knowing when an index or plan (or better package) has last been "accessed".
For Indexes :
If you only have static SQL and all your programs have been explained you could check the plan_table. But even then Indexes that are needed for R.I. could be not in the plan_table but still be very needed for the performance of your application.
In DB2 9 there is a column LASTUSED in SYSIBM.SYSINDEXSPACESTATS.
For plans and packages :
You should know which programs are running on your system based on info outside of DB2. Based on that you would know which packages are still needed.