Can I delete a custom subtab that was added to the classic UI?

In the classic UI, you can't delete a custom tab after your sandbox is published, but you can hide it.

Here's how:

1. Create and enter a sandbox with Application Composer as one of the tools.

2. In Application Composer, select the object.

3. Expand the object in the object tree, and select the Pages node.

4. Select the Application Pages tab.

5. Find the type of page that you want to modify, such as the details page layout. You can create a new custom layout by clicking the Duplicate Layout icon to duplicate and edit an existing layout.

6. In the layout Subtabs region, click the Hide, Show, and Reorder button (the icon with the two vertical up and down arrows)
7. Hide the subtab.
8. Click Done.
9. Publish the sandbox.