Making items on a layout non-editable

Content (please ensure you mask any confidential information):

I would like to make a custom field (txt) placed on the page layout non-updatable.
However, when I try to set the above, it seems that there are some things I can set and some things I can't.
For example, it appears that you can set it for custom fields created on custom objects, but not for custom fields created on activities.

The specific steps are as follows:
1.Application Composer>Activity>Pages>Task Details>Custom_Layout
2.Click on the custom field(text) placed on the Custom_Layout
3.The Updatable in Layout Properties is fixed at 'Y' and cannot be changed to 'N'