Currently, arcpy.mp supports Spatial Map Series (MapSeries class) and Bookmark Map Series (BookmarkMapSeries class) but the third and final type of map series, Thematic Map Series, is not supported. For a layout that contains a Thematic Map Series, the Layout.mapSeries property returns None, as if the layout had no map series at all.
My suggestion is to a ThematicMapSeries class along with other functionality to interact with Thematic Map Series equivalent to the other two types.
My use case is that I have a python notebook to batch export layouts. It can handle layouts containing Spatial or Bookmark Map Series but since arcpy.mp is unaware of the existence of Thematic Map Series, layouts containing those have to be exported manually.
I am not the first to request this (For example see https://community.esri.com/t5/arcgis-pro-questions/exporting-thematic-map-series-with-arcpy-mp/m-p/1557775) but as far as I can tell nobody has posted it to the ideas forum until now.