Plain-text files are actually not widely used outside of software engineering and when they are, they are used like binary and only edited with graphical editors. And most non-tech people don't see the benefit if it is there - in software development versioning was born from pain: big source trees are unmanageable withoud SCM. For typical office documents you miss some benefits but can live without.
One interesting anecdote / micro case-study: In my family we have a vacation home that we rent out. My mom (>70 years old, completely non-technical) manages the renting and updates an online calender where guests can see which dates are available at what price. I developed a very simple custom text format and a static site generator an my mom changes the data using the web-ui of my gitlab instance. I had to teach her but now she is very happy with it and much faster than with the previous solutions. And git helps greatly because mistakes do happen and its very easy to revert and see when they happened etc..
Wow, this is very interesting anecdote and case study for Git based user application. This can be an excellent Local-First kind of software that can work for and appreciated by the non-technical people. Are you opening up the custom software or is there any equivalent of available software that can perform similar to this scenario?
I don't have real plans to open this up as it is very specific for our use-case - perhaps I might clean it up a bit when I have time and put it on github. I don't know of any equivalent software.
It's interesting that you describe your use case as "very specific" now, but earlier it sounded like a thing that that thousands of other people might do similarly. What makes you not want to do things their way; what makes them not want to do things your way? It sounds kind of like Airbnb/Vrbo but without the expense and protection of a well-paid middleman, in which case I'd expect lots of people to be interested.
conceptually many people might want to do it the same way. But the apartments and pricing model (different seasons) etc. is hardcoded.