When applying a filter based on a field with an assigned domain, the values that have been filtered out is still remain in the web map's legend.
Last Published: September 3, 2015ArcGIS Online
Bug ID Number
August 4, 2015
August 21, 2020
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
It was a design decision to show all coded values if there are domains setup on the field when creating a type renderer.