Even though the operating system is backed up, you’ll actually get a choice when restoring whether or not to restore the OS as well. This means that you can restore the backup to a new MacBook Pro that only supports Mojave for example. All your personal files, applications and preferences will be restored. However system specific items such as for example hardware drivers will not be restored.
You can find instructions on how to restore excluding the OS here:
If you do intend to restore with the OS, please be advised that even though the El Capitan system requirements, that you have been provided a link for, specified “or newer” when talking about the MacBook Pro - that is strictly not true. It was true at the time that document was written though.
If you buy a recent MacBook Pro 2018 with TouchBar, it does not support El Capitan.