If you are intending to conduct a Fair Lending Risk Assessment (which is strongly advised given that regulators are expecting banks to be monitoring their Fair Lending risk) you not only should be collecting the reasons for denial, you should be collecting and compiling credit score, LTD, DTI, etc. so you can conduct an automated Fair Lending Risk Assessment. The time is coming when it will be mandatory, but even now examiners are expecting a risk assessment to be conducted and it is impossible without knowing and maintaining the reasons for denial. So my advice is don't stop collecting and reporting the reasons for denial.
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