We know that we need to have a purpose to keep a record and that there are retention periods for records etc.  But what about expired warnings, disciplinaries and grievances and what not.

Unconscious bias is a concern if people are seeing expired warnings and historic information about a person.  What if that was to do with the relationship they had with their manager at the time and now, several years later another manager is in place.  For as much as we try, we are influenced by what we see and hear. 

So how can businesses try and protect people so that they are not discriminated against for something that happened in the past and is now resolved?

Think about where records are stored, who can access them and how long to keep them.  How long does a business want to keep an expired warning?  Some businesses keep this information for a year, some keep it indefinitely and it forms part of the employment records which are then kept for six years after a person has left a business.  What is there to gain from keeping the records?