There were all kinds of standards, guidelines that made people recognize what the program is doing and how to operate it. Nowadays, UIs are mostly defective, trying its best to hinder effective usage and hide/strip functionality. There are of course progress too, but something was lost that make applications today much less intuitive and hard/tough to use.
Also they were called GUI standards or UI at the time.
The modern equivalents called UX isn't reflecting the same conglomeration of standards and conventions though. So not talking about the newer stuff.
I'm no expert on it, and it required specialized expertise. It's been abandoned for mobile interfaces and the modern UX stuff, which often optimizes for design over functionality.
If you've never used old software, it's hard to explain. But old Apple or Microsoft GUI standards would cover the basics, but you'd also need to study the applications and how they presented their GUI.
While I broadly agree with the UX/HIG/design guideline issues that are common in modern software... literally none of them have anything to do with the technicals of how quickly the UI reacts to actions and renders the next state. You can have responsive UI in all of them. And all the modern ones also say "jank is bad, don't do that".
Back in the days™ there were widget libraries as part of the OS, which followed the guidelines by the OS vendor. This gave a foundation for somewhat similar behavior and standardisation of behavior. This gives usability.
Nowadays man's applications are web apps, build without such frameworks, with less UI research and even where frameworks are used they are often built with a somewhat mobile first approach.
One made up example that attempts to embody the complaint:
If I visited a site dedicated to hamburgers today, I would not be surprised if the "Log Out" button was presented as an image of a hot dog. It would be a mystery to me what that hot dog did until I clicked on it.
Compare this to 90's UI, where it would pretty much be unheard of to do something like that. It would have been a joke, or a novelty. These days that sort of ambiguous interface isn't presented as a joke or a novelty - it's the real product.
