Bug or feature : be_filter is hidden if attribute type Textbox

[mrothtrc]

I would like to filter the custom attribute "Vendor (Hersteller)" based on all existing values in the tl_iso_product table in the backend. The option be_filter ("Filtern im Backend") is only displayed if the attribute type is "Select-Menu" but you need to use the Option Manager or Table with Foreignkey this isn't what I want.

Workaround :
Update the attribute in the mysql table tl_iso_attribute and set be_filter = 1. All existing values will be displayed in the backend as a Select Menu. And you are able to filter for vendors

Bug or Feature Request : Please enable the option be_filter for the attribute type "Textbox"

Picture shows the modified attributes based on table values :