In my previous post, I wrote about how sharepoint checks if a field is groupable. One of the things the code there does is check if a field is sortable.
I had assumed that that meant that if a custom field has in it's schema "Sortable=False", then it would not show up in the "sort" and "group by" dropdowns in the edit and new view pages.
However, it turns out that if the custom field inherits from a field that is sortable (like the text field for example), then what you set in the schema gets ignored - and the field still returns "true" for .Sortable.
I have not found why that is, and the only workaround of how to make a custom field unsortable if it inherits from a text field is to change the code in ViewNew.aspx and ViewEdit.aspx pages to skip those field types.