IFF is still relevant even now if you want a really straightforward binary blob package. There's almost nothing to it for basic parsing - just watch out for the byte alignment padding and endianness, and default to loading chunks by name, not order.
(On that note, a lot of software that works with IFF/AIFF/RIFF will "do it wrong" and expect a particular chunk order for the data it works with, which over the years has led to a lot of "fix-up" software for cranky programs needing massaged data.)
It often slips my mind that Electronic Arts was once one of the premier Amiga software publishers, specifically developing Deluxe Paint. It's weird to think of any of the old Amiga-related companies surviving and thriving decades later, because most didn't. The ones who have succeeded the most have long since lost any resemblance to the original company; EA is one of those.
I remember studying IFF, and specifically the AIFF audio format, for some software ideas I had back in the late 80s or early 90s, but never actually made into something useful. AIFF, in an extended form, is still used in Apple audio products (like GarageBand and Logic). So, IFF is still around, to some extent.
IFF is a very well-designed format. While it is not directly used anymore, variations on it still do: RIFF, which is little endian as opposed to IFF which is big endian, lives on in WAVs, AVIs, and other formats. AIFF, an Apple audio format; Even PNG and Apple's QuickTime container format (commonly known as MP4 these days) were inspired by IFF. [0]
Using FourCCs[1], which seem trivial and intuitive these days, was essentially introduced in IFF - back in 1985 when it was, the prevailing encoding was "just enumerate them in a table written on a napkin somewhere".
IFF was the prototype for future-proof binary tag-length-value formats.
Where was it used? I worked there from 2002-2004 and I don't recall it being used, though I don't doubt it was used somewhere. The teams were pretty silo'd.
My experience there was mostly memory dumps that were byte swapped in ad hoc ways, sometimes on the tool side and sometimes at runtime. I always tell people the story of the 8000 line file with 2 functions. 4000 lines to read a data structure, and 4000 lines to write it... all done with for loops, and fread/fwrite.
No, the "packed" file of related resources in a single filesystem file has never gone away. Very popular in games (WAD, BSP, Steam games etc), but also used in video container formats - you wouldn't ship audio and video as two separate files!
(On that note, a lot of software that works with IFF/AIFF/RIFF will "do it wrong" and expect a particular chunk order for the data it works with, which over the years has led to a lot of "fix-up" software for cranky programs needing massaged data.)