We are looking to restore some critical data from an old backup tape, which Symantec BackUp Exec 2010 R3 was used originally to back up the data. Can Veritas BackUp Exec 21 read and retrieve the data from a Symantec BackUp Exec 2010 R3 tape?
Generally, Yes, if it is files you are restoring.
If you are wanting to restore a database, or anything from an agent, it gets more complicated as you must have the proper environment to target the restored data. That gets into possible issues with unsupported OSes or systems that you may not have running any more.