It's interesting this is making a revival in C for efficiency. When I was first learning to code ('90s), this style of "parallel arrays" was seen as a scripting-language wart. When you semantically want an array-of-struct in a bash script, you resort to parallel arrays instead, because it's what you can do easily.
> When I was first learning to code ('90s), this style of "parallel arrays" was seen as a scripting-language wart.
When I first learned to program in the 80s, this was known as "Fortran order" (as opposed to Pascal or C order), and in the later nineties, I think that "column major" and "row major" became popular, and they still are.
This goes back to sometimes in the 60's, with APL and Fortran doing column major on purpose, BASIC by accident, and most of the rest doing Row major.
> this style of "parallel arrays" was seen as a scripting-language wart
It was only a wart because they didn't support compound types at all (and in some cases they did support compound types but people were not taught them when moving to a better language and kept their old techniques). It is compounded by the fact most scripting languages are untyped, so structure packing (the main justification for using arrays like that in more modern languages (for general memory use, stack size, or cache-hit optimisations)) isn't nearly as relevant.
Basically if you see parallel arrays used without a comment stating that it is done for packing reasons or similar, it is there either because the language does not support compound structures well or because the programmer knew no better. In a language where structure packing can be useful, it is a perfectly valid optimisation (though for small data not in tight loops, it may be an optimisation too far).
