That’s the key.
You cannot decide on that when you have no plan on you smart home.
If you want to just switch “normal” fixtures, 230v should be sufficient, but if you want to do more clever stuff with multple switched, a microcontroller at the wall switch can be present and can require only 24v.
For what I know, there is no real guideline for a smart home.
I enjoyed this post and maybe can help you start some things:
When I moved houses a year ago, I did not had the opportunity to re-do the wiring, but I had the option to go for some “standards”.
So I decided on google smart devices
Shelly switches over wifi or ethernet where possible.
Philips Hue for lightning.
In this way I can mess around with some automations, use global API’s, but my house is not down when internet is gone. So still can use the lighting with swithces/hue buttons, monitor my Solar Inverter and control all the appliances.
Not getting a notification when somebody is at the door is just not that important. Everybody has a phone an can give me a ring when I don’t see it.