I currently use a tiling wm (bspwm) with plasma 5.1 and nearly everything works fine. However, it came to my attention that KDE5 is adopting Wayland in place of X11. Once KDE switches to Wayland, will it still be possible to use X11 window managers with Plasma 5.x?

kde will continue to support x11 for quite some time (years) and you cannot use an x11 wm as wayland wm, since it requires x11...