I have a model that uses the merge tool to merge 2 feature classes together. I have set the field mapping parameter to exclude 4 fields from one feature class and 4 fields from the other but the end result still has those 8 fields in the output. I attached screenshots of the parameter set up and the output fields. Note that one field is joined after the merge is performed.
Using ArcGIS Pro 2.3.2 on Windows 7