Whenever I dig into this I discover that what people complaining really want to do is modify their cars so it no longer meets emissions standards (you can get more power and/or better fuel mileage by doing this). Nobody is replacing ECUs with one of their own design that otherwise meets emissions. Sensors and parts are easy to replace (sometimes at high cost), and mostly radially available. The OEMs already tell mechanics what all the diagnostics codes mean.
sure most of us reading this have the skills to write new code for their ECU, but realistically almost none of us would do that anyway unless we want to make a trade off that effects emissions.
OEMs only tell authorized mechanics what the codes mean. Most cars have the basic standardized OBD-II codes, and an additional much-more-useful set of codes/diagnostics that are proprietary.
They tell the third party scan tool manufactures as well. (for a "reasonable" price - where reasonable can be 6 figures) I used to work on those scan tools, and so I saw the data (I probably shouldn't talk about it - lets just say were reversed engineered their tools often).
Many mechanics will read and tell you the codes for free. Auto part stores will as well.
It's not just that. For example, BMW has paired the main battery to the ECU in their cars for years. So a dead battery requires a trip to an authorized repair shop (or buying an OBD scan tool) to have a new battery operate correctly. This is relatively easy & safe ~4 bolt job (2 tether the battery, then 1 for each terminal) in many other cars. Technically, it can be self-repaired, but it's extra steps and cost because reasons.
sure most of us reading this have the skills to write new code for their ECU, but realistically almost none of us would do that anyway unless we want to make a trade off that effects emissions.