Prevent the slide-out from appearing in fullscreen safari
There isn't a way to stop this from happening with conventional methods (i.e. through settings or modifying a .plist file.) It has also been around since Safari 9 from what I can glean from my researches. Search on this site, and in Google in general, to find ways to 'trick' Safari by moving the Dock to the left side of the screen, changing some parameters through a defaults write command to a .plist file and setting up some trackpad swipes and whatnot. Complicated and ultimately a kludge.