You might be making yourself disposable specifically in the context of your original role which you are basically making redundant by documenting and automating everything - but trust me any sensible business will want to keep a staff member who massively improved their team's productivity (by enabling them to be more independent with less silo'd knowledge) and reduced the business' risk exposure (by making potentially critical knowledge more accessible and reducing single points of failure).
Employees who try to become indesposable by turning themselves into a mega-silo of knowledge that nobody else in the org has might gain some job security in the short term but they lose out on any potential job progression.
Turning yourself into a silo like this is practically blackmailing your employer into keeping you. They might keep you employed because they need to keep their systems online, but they will also not think twice about replacing you as soon as an opportunity is presented. That is making yourself disposable.
Turning yourself into a leader who improves the outcomes of various teams in a business will not only make you indespensable, it's genuinely the best (if not the only realistic) path for an engineer to work their way up into more senior or C-suite positions.