ImGui is amazing to work with though. Like holy fuck is it pleasant compared to basically every other UI-development paradigm ever in the history of user interfaces.
It is like static linking, it was also there decades ago.
There are reasons why the world has moved on from those approaches.
And OTH, event-driven UIs have also been around as long as UIs exist, that makes them at least as 'outdated' as immediate mode UIs.
Some of them keep being rediscovered in endless loops of fashion.
The reason for this is often that the environment (mainly the hardware) has changed so much that it may make sense to look into discarded old ideas again. For instance, dynamic linking was extremely important in the age of slow floppy discs and when RAM was counted in kilobytes, but those environmental factors are no longer an issue, and the advantages of static linking outweigh the disadvantages of dynamic linking again.
And those “reasons” often SUCK; that is the point.
It feels intuitive and that's why it's the de facto standard for building debug tooling. But this intuitiveness comes at a price! No matter how hard you try, this will always be slower and computationally heavy compared to retained mode.
Great for some stuff, terrible choice for some other.
immediate mode rendering and immediate mode gui are unrelated. immediate mode gui libraries generally dont use immediate mode rendering because its realy slow.
immediate mode gui tends to be faster in practice because you just naturally end up with less code. also its very easy to write a retained mode ui ontop of an immediate mode gui library.