My journey is currently running with contacts going through the first steps. I would like to add a decision split as a last step in the journey. Will the contacts currently in my journey go through that decision split added after starting the journey?
You can only edit the flow by creating a new version of the journey. This will allow you to add a decision split in your flow.
You do need to observe, that by editing a new version, you will not affect contacts being in the current version. This means, that someone currently being in one of your two first wait steps, will continue on the journey path in version 1 (not having the decision split).
Any contact entring version 2, after you activate it, will be injected into the journey with decision split, and travel through the journey path accordingly.