Are you thinking here of pre-multitasking desktop usage, stuff like DeluxePaint, Scream Tracker, that kind of thing?

Certainly the late 90s was the heyday of desktop consistency on Windows, in the 95/98/ME era, I think driven largely by the conventions Microsoft established in Office. And I believe Mac OS gave pretty good platform-level guidance then too, so things were generally okay with a few exceptions— stuff like media players that have always been more on the fanciful side.

It is my recollection as well. Most applications used VB, Delphi, MFC etc. that all had the "native" OS look and feel. There were some exceptions like WinAmp and others, but from what I can remember most applications were more consistent than today.

Early 90's, especially on Amiga -- where a proper windowing desktop env ("workbench") coexisted with the wildest custom UIs. Maybe it was the roots of the machine, heavily used as a games console and a demoscene workhorse? It seems like at that time there was so much creative design effort put into UX -- and it didn't seem to get in the way, maybe because each genre of software was kind of on the cutting edge back then, establishing what would eventually evolve into conventions. Mod trackers, image editing, disk copying, etc. Maybe it's a bit of nostalgia, but it felt really immersive to pull up a piece of software you were familiar with; each UI was so distinct and purpose-built, but it also had.. flourish? style? soul? Not so much now.

Nah, the Windows ecosystem never even got close to being consistent.

MS Office had its own UI toolkit and routinely invented new UI paradigms that weren't exposed in any Windows API, leaving people who wanted to look native scrambling to reimplement. This was particularly the case for toolbars. MS Office first invented the so-called "coolbar" and then the ribbon. Internet Explorer also rolled its own toolbar styles in ways not supported in the base Windows API e.g. toolbars with large icons and sliding sub-sections. Inventing custom toolbars was practically a sport on Windows; Netscape also did it.

At the time the most popular media players were WinAmp (totally custom and themeable to boot), RealPlayer (custom UI https://andrewnile.co.uk/blog/remembering-realplayer/), Quicktime (custom UI) and Windows Media Player (mostly but not entirely native).

Even the base utilities that came with Windows weren't consistent with each other. It wasn't uncommon in the Win 9x era to find programs still using Win3.1 style file dialogs ... a few are still buried in Windows today!

The problem got worse when you examined the artwork. The stock icon library in Windows was anemic, so dev platforms frequently had to expand the core library with their own. Delphi apps could be easily identified by the distinctive icons in their buttons (https://zarko-gajic.iz.hr/wp/wp-content/uploads/2017/02/delp...).

Restyling window decorations was also very common. Microsoft themselves did it routinely, for example their flagship Encarta encyclopedia app had totally custom widgets and window styling: https://winworldpc.com/product/encarta/1999

To get online most users were running something like CompuServe (custom web-style main UI https://thedayintech.wordpress.com/wp-content/uploads/2013/0...), or AOL (custom UI https://www.reddit.com/r/nostalgia/comments/ehxb1g/the_aol_h...), or MSN (custom UI https://cdn.prod.website-files.com/6179a66d5f9cc70024c61878/...)

Windows apps of this era were much like web apps are today: they shared some common code for things like rendering menus, buttons or widgets in their settings screens, but the main UI users interacted with were almost always custom widgets that were extremely varied between apps. Win32 was nearly impossible to style compared to HTML so this represented a large investment of developer time, but a custom branded UI was believed to be worth nearly any cost. This is something fundamental to how humans work and is pointless to fight, a lesson the web platform fully embraced giving it an advantage over other UI toolkits of the era.