The interface used to have a button to insert a horizontal rule (a.k.a. a horizontal line). Very useful for putting a visual separator between sections of text. It allowed users to make their emails look more professional.

I think it simply inserted the HTML <hr> tag. So, very easy to implement.

Why would you get rid of such a nice feature? Bring it back!

Excellent features should not be eliminated in the interest of dumbing down the interface.