This is one of those things that fits more naturally into Themes, as a design choice, rather than a core feature but we’re always watching and listening so never say never 
As we’ve heard in the thread already several themes include a popover.
For those who don’t have it in their theme, you can add something via code injection.
There are services that offer a side-loadable widget via a line of JS code. https://popupsmart.com/ is mentioned around the forum a lot.
My preference is to use a few lines of JS to load the Ghost portal. It just seems the all-around lightest (on the pocket, on the page) option. This post on the forum gives a good solution: How to connect a pop-up solution to collect emails for newsletter? - #3 by jtraverso
I implemented it here so you can see it working: Simple "sign-up" pop-up for Ghost