As someone who shipped my fair share of critical production features, I find this plan raising my eyebrows somewhat. Disabling a feature AND simultaneously removing the codebase for that feature almost never ends well. There will always be some use cases that people haven't thought of.
In serious projects (read, your career is at stake) a much better strategy is to first make the feature unavailable by normal means while still allowing a workaround (in this case, for example, PLM could remove X11 option from the menu but still allow X11 sessions when some magic environment variable is set.) That would give people an easy way to get the old functionality if something is critically impaired for them. And only then, once we are confident that no massive unforeseen issue has surfaced, can the codebase be removed.