Also the term open is very relative here (you have to pay a fee (1000 euro + VAT last time I checked) to access the standard, naturally there are no redistribution rights, KNX certification process is order of magnitude more expensive.
You need a closed source configuration software to be able to configure the devices. Runs only on Windows (XP did it last time but things may have been changed). A single licence costs 1000 euro + VAT, unless you use limited version that works up to 20 devices and costs 100 euro + VAT (in many small cases that would be enough as the limitation is project based).
PS. It is rather an attempt to put things into a contexts than a criticism.
Minimum setup:
1) KNX power supply (provides power for the KNX bus),
2) KNX actuator (relay or dimmer or special (say LED stripe)),
3) KNX switch (or an input device (can use standard wall spring switches)),
4) KNX bus connector for programming (USB tongle, KNX/IP router, there are many options),
5) KNX configuration software,
6) Windows machine (VM works too).
This setup allows to map the actuator and switch in the configuration software and upload the configuration into the devices.
Please also note that encrypted communication between KNX devices is commonly not supported (but so is it also for a simple pulse signal from the wall switch to the relay).
Took me a little while to find out what is was actually about on the website, most pages contains just marketing speak.
I wonder if there is any other (really) open standard that could be useful to follow in DIY systems.
Future modifications and changes will be easy even with support discontinuation from our side. Every part of the system is modular - you can pull out RPi with custom modules and place any PLC instead.
You practically never pay the 1000€ because it is owned by the company putting the cables in your home. But, yes, you have this software fee if you want to change the hardware on a regular basis (but I must say, I don't know anybody doing this).
Sorry, but no. The reason to DIY is that you can continue to maintain it properly as you go along.
[0]: https://knx.org