I have just been diagnosed with Dementia. My Dr. Says I have to inform my work place to go on modified duty. Due to safety. What is the legal end if I don’t inform my work place

If you don't inform your workplace then the company will not have any obligation to accommodate your medical issues since they will not be aware of them. If you are ultimately terminated due to something that was affected by your medical issue then you cannot later claim discrimination if you don't advise the company.