As a medium to describe systems it was harder to understand that just a flowchart, and less specific than just reading the code code. A useless halfway house.
Also it is quite useful to understand sophisticated class dependency diagrams.
We mostly quit doing UML because for most people, most of the time, it mostly wasn't worth it. It didn't provide enough value to be worth the effort. So what replaced it was usually nothing.
I'm imagining something on an iPad or Android tablet that lets me just draw my usual boxes, circles, lines, terrible handwriting, and so on, and then I can punch a button and get it as a draw.io (or whatever) compatible diagram...?
The Newton handheld had some abilities along this line in the mid nineties so it seems like it shouldn't be an impossible dream.
In my experience, the best approach in documenting software systems is the C4 model: https://c4model.com/
Also powerpoint equivalents -- For more control over format and presentation.
They can serve as example and documentation on how the architectural pieces fit together in a piece of software.
That plus high level, loosely specced diagrams can encapsulate most simple/medium complexity apps.
On the whole I think that the value of "diagrams as the source of truth" turned out to be almost non-existant for much the same reasons that mostly we prefer source code over visual programming environments.
Flowcharts were never all that useful (to me at least) because they were harder to read than the pseudocode!
I use draw.io (diagrams.net) or Miro depending on what's in commonest use at the current Org for informal data flow diagrams and the like. Does Microsoft have some kind of Visio descended monster in Office 365? I hope I never have to find out!
I use PlantUML¹ for sequence diagrams² and state diagrams. So much so that I wish GitHub and its brethren would support PlantUML natively in their flavours of Markdown.
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¹The IntelliJ plugin is mercifully good
²Or swimlanes.io is quite good for these, especially for sharing ephemeral stuff
