How do you know this culture existed 20 years ago when this system was developed? It is almost certain that a corporate developer in the 1990s would be 100% waterfall.
The issue is very often related to massively complex corporate requirements (the Post Office makes me cringe, even today, with the complexity of their postal system) and then coupled with the ever-present need to keep costs low, especially when designing something so complex.
I doubt anyone building this thought it would be OK to break things!
You are correct. I've seen the Horizon design documents - they are very detailed, thorough and complete. The system was not designed in an Agile manner of iterative design / build first - document later.
The issue is very often related to massively complex corporate requirements (the Post Office makes me cringe, even today, with the complexity of their postal system) and then coupled with the ever-present need to keep costs low, especially when designing something so complex.
I doubt anyone building this thought it would be OK to break things!