Thanks for the context! I didn't research OpenTherm much, since my boiler is one of the most basic models on the market. It supports just the bare minimum from V2 spec, but it works, so I'm not complaining :) This boiler was already installed when we bought the house, so not needing to replace it was a great way to save some money for remodelling.
Otherwise, I completely agree that charging 3K for the spec is a stupid money grab. When the time comes to build a new house and choose a new boiler, my choice will be influenced by what protocols are available and whether I can control the boiler with my own thermostat.