Proprietary software is software that is owned by someone and was specificially developed for that one rights holder, in this case, the software is part of Plesk, hence the Plesk company is the rights owner to that software. There is no script that you can take, modify or re-use, it is simply something that was built into the Plesk panel software. So from the user's perspective there is no such thing like "the script of the Cookie opt-in".