Minecraft Skin Creator

I've been using a custom skin for a few months now and every so often I have to remake it because Minecraft will reset it. This has never usually been a problem with me up until recently when I found a setting missing. When designing my character I use the Redstone Arm as my left arm and when doing that I noticed that I could no longer switch to a different arm and instead it changes both arms. Where did this setting go? Is this the work of a bug or did you just straight up remove the setting??