I like the horizontal line feature to segregate two areas of my email to send to boss to review the information below the line.
I use this feature frequently as well for sending folks information they need to review; would love to see it added back in for new Outlook.
In new Outlook, my fix has been to I add in a 1x1 table, format it for about 1/3 of the width of my window, and add a bottom boarder - it isn't the most ideal but it works for me for now. Plus that is faster than trying to swap back and forth between classic & new Outlooks as I'm trying to work in new outlook to prep some guides for our teams on its functionality before it gets pushed out by our IT team.