3426929 - Is it possible to track changes that have been made in business rules?

You want to check all changes that have been made to the business rules, including which fields and settings have changed.

SAP SuccessFactors Employee Central
SAP SuccessFactors Platform

You can also check the history of the rule in Configure Business Rule. But it is currently not possible to audit business rules at field level.
The audit is only possible at the header level. 

This is an expected behavior based on how system is designed to work.
In case it does not cover your business needs, you can submit an enhancement idea by following the KBA: 2090228 - How to submit enhancement ideas for SAP SuccessFactors Products steps.

Employee Central Object and Element Audit Configuration;
How to submit enhancement ideas for SAP SuccessFactors Products;

audit, business rule, business rule changes, report business rule, field rule report, audit rule configuration, business rules, employee central, configure business rules changes, , KBA , LOD-SF-EC-JOB-RUL , Job Info Business Rules , Problem