Guess that depends on the individual(s) in question. I've done both maintain friendships with past girlfriends and also have gone our separate ways with others. Guess it also depends on how emotional the split was, less wild emotions probably will equate to a maintained friendship.

To each their own. If it's particularly painful to separate, but we both agree it's for the best, then cutting contact is the most appropriate thing until the attachment dissolves and we're able to rejoin each other's lives in friendship. If things became toxic, or violent.... no contact ever again. In my experience with the latter, rarely does that toxicity disappear in those people. They merely learn to hide or mask it better with each new relationship, and the realization that they may need to change/grow comes to them incredibly slowly.